How to Build a Sudoku Solver Android App?

In this article, we are going to make a sudoku solver Android app in Kotlin and xml. The main focus of the app will be on the sudoku-solving algorithm so the the design part has been hardcoded. A sample video is given below to get an idea about what we are going to do in this article.

Step by Step Implementation

Step 1: Create a New Project in Android Studio

To create a new project in Android Studio please refer to How to Create/Start a New Project in Android Studio. Note that select Kotlin as the programming language.

• Now we will create the layout of the sudoku solver using Table Layout, you just need to create one row and copy paste that 9 times and give the ids as shown.
• Although you can reduce this redundancy using recyclerview but to keep it simple I have Hardcoded the table.

Add viewBinding in builde.gradle(app:Module) inside android{} and sync the files.

Kotlin

viewBinding{
  enabled = true
}

Step 2: Working with the activity_main.xml file

Navigate to the app > res > layout > activity_main.xml and add the below code to that file. Below is the code for the activity_main.xml file. Comments are added inside the code to understand the code in more detail.

XML

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="15dp"
    tools:context=".MainActivity">

    <TableLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:padding="5dp">

        <TableRow>

            <EditText
                android:id="@+id/a_00"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_01"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_02"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_03"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_04"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_05"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_06"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_07"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_08"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

        </TableRow>

        <TableRow >

            <EditText
                android:id="@+id/a_10"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_11"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_12"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_13"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_14"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_15"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:inputType="number"
                android:gravity="center"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_16"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_17"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_18"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

        </TableRow>

        <TableRow>

            <EditText
                android:id="@+id/a_20"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_21"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_22"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_23"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_24"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_25"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_26"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_27"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_28"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

        </TableRow>

        <TableRow
            android:layout_marginTop="16dp">

            <EditText
                android:id="@+id/a_30"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_31"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_32"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_33"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_34"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_35"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_36"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_37"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_38"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />
        </TableRow>

        <TableRow>

            <EditText
                android:id="@+id/a_40"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_41"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_42"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_43"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_44"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_45"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_46"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_47"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_48"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

        </TableRow>

        <TableRow>

            <EditText
                android:id="@+id/a_50"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_51"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_52"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_53"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_54"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_55"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_56"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_57"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_58"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

        </TableRow>

        <TableRow android:layout_marginTop="16dp">

            <EditText
                android:id="@+id/a_60"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_61"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_62"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_63"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_64"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_65"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_66"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_67"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_68"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

        </TableRow>

        <TableRow>

            <EditText
                android:id="@+id/a_70"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_71"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_72"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_73"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_74"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_75"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_76"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_77"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_78"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

        </TableRow>

        <TableRow>

            <EditText
                android:id="@+id/a_80"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_81"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_82"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_83"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_84"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_85"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number"
                android:layout_marginRight="22dp"/>

            <EditText
                android:id="@+id/a_86"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_87"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

            <EditText
                android:id="@+id/a_88"
                android:layout_width="0dp"
                android:layout_height="45dp"
                android:layout_weight="1"
                android:gravity="center"
                android:inputType="number" />

        </TableRow>

    </TableLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:layout_marginTop="32dp">

        <Button
            android:id="@+id/solve"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:background="#283593"
            android:padding="5dp"
            android:layout_margin="8dp"
            android:text="SOLVE"
            android:textStyle="bold"
            android:textSize="20sp"/>

        <Button
            android:id="@+id/clear"
            android:layout_width="0dp"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:background="#283593"
            android:padding="5dp"
            android:layout_margin="8dp"
            android:text="CLEAR"
            android:textStyle="bold"
            android:textSize="20sp"/>

    </LinearLayout>

</LinearLayout>

Step 3: Working with the MainActivity.kt file

Go to the MainActivity.kt file and refer to the following code. Below is the code for the MainActivity.kt file. Comments are added inside the code to understand the code in more detail.

Inside MainActivity class declare the binding variable so that it can be used globally

Kotlin

private lateinit var binding: ActivityMainBinding

Inside the onCreate method, we will use a 2d array of string to store the input and a 1d array that contains all the ids of editTexts.

Kotlin

super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val matrix = arrayOf(
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
        )

        var arr = ArrayList<EditText>()
        binding.apply {
            arr.addAll(
                arrayOf(
                    a00, a01, a02, a03, a04, a05, a06, a07, a08,
                    a10, a11, a12, a13, a14, a15, a16, a17, a18,
                    a20, a21, a22, a23, a24, a25, a26, a27, a28,
                    a30, a31, a32, a33, a34, a35, a36, a37, a38,
                    a40, a41, a42, a43, a44, a45, a46, a47, a48,
                    a50, a51, a52, a53, a54, a55, a56, a57, a58,
                    a60, a61, a62, a63, a64, a65, a66, a67, a68,
                    a70, a71, a72, a73, a74, a75, a76, a77, a78,
                    a80, a81, a82, a83, a84, a85, a86, a87, a88,
                )
            )
        }

Now when the user types in the input integers and click solve then we have to first check for the validity of sudoku, so continuing the above code in onCreate

Kotlin

binding.solve.setOnClickListener {
           var k = 0
           for (i in 0..8) {
               for (j in 0..8) {
                   if (arr[k].text.isNotEmpty()) {
                       matrix[i][j] = arr[k].text.toString()
                   }
                   k++
               }
           }

           if (!isValidConfig(matrix)) {
               Clear(matrix, arr)
               Toast.makeText(this@MainActivity, "Invalid Sudoku", Toast.LENGTH_SHORT)
                   .show()
           } else {
               solve(matrix, 0, 0)
               k = 0
               for (i in 0..8) {
                   for (j in 0..8) {
                       arr[k].setText(matrix[i][j])
                       k++
                   }
               }
           }


       }
       binding.clear.setOnClickListener {
           Clear(matrix, arr)
       }

Now write the below code outside onCreate for validity check

Kotlin

fun isValidConfig(arr: Array<Array<String>>): Boolean {
        val n=arr.size
        for (i in 0..n - 1) {
            for (j in 0..n - 1) {
                if (arr[i][j]!="" && !valid(arr, i, j)) return false
            }
        }
        return true
    }
private fun valid(arr: Array<Array<String>>, row: Int, col: Int): Boolean {
        return (arr[row][col] !in "1".."9") && notInRow(arr, row) && notInCol(arr, col) &&
                notInBox(arr, row - row % 3, col - col % 3)
}
fun notInRow(matrix: Array<Array<String>>, row: Int): Boolean {

        val st = HashSet<String>()
        for (i in 0..8) {

            if (st.contains(matrix[row][i])) return false

            if (matrix[row][i] != "") st.add(matrix[row][i])
        }
        return true
}

fun notInCol(arr: Array<Array<String>>, col: Int): Boolean {
        val st = HashSet<String>()
        for (i in 0..8) {

            if (st.contains(arr[i][col])) return false

            if (arr[i][col] != "") st.add(arr[i][col])
        }
        return true
}

fun notInBox(
        arr: Array<Array<String>>,
        startRow: Int,
        startCol: Int
    ): Boolean {
        val st = HashSet<String>()
        for (row in 0..2) {
            for (col in 0..2) {
                val curr = arr[row + startRow][col + startCol]

                if (st.contains(curr)) return false

                if (curr != "") st.add(curr)
            }
        }
        return true
 }

// on clicking the clear button to clear sudoku
 private fun Clear(matrix: Array<Array<String>>, arr: ArrayList<EditText>) {
        var k = 0
        for (i in 0..8) {
            for (j in 0..8) {
                matrix[i][j] = ""
                arr[k].setText("")
                k++
            }
        }
    }

If the sudoku is invalid toast will be printed else if it is valid below code will be executed

Kotlin

private fun solve(matrix: Array<Array<String>>, i: Int, j: Int): Boolean {
        if (i == 9) return true
        if (j == 9) return solve(matrix, i + 1, 0)
        if (matrix[i][j] != "") return solve(matrix, i, j + 1)



        for (it in 1..9) {
            if (isvalid(matrix, i, j, it)) {
                matrix[i][j] = "$it"
                if (solve(matrix, i, j + 1)) return true
                matrix[i][j] = ""
            }

        }
        return false
    }

    private fun isvalid(matrix: Array<Array<String>>, i: Int, j: Int, value: Int): Boolean {
        val row = i - (i % 3)
        val col = j - (j % 3)

        for (x in 0..8) if (matrix[x][j] == "$value") return false
        for (y in 0..8) if (matrix[i][y] == "$value") return false
        for (x in 0..2) for (y in 0..2) if (matrix[row + x][col + y] == "$value"
        ) return false
        return true

    }

Complete MainActivity Code:

Kotlin

package com.example.sudokusolver20

import android.os.Bundle
import android.widget.EditText
import android.widget.Toast
import androidx.appcompat.app.AppCompatActivity
import com.example.sudokusolver20.databinding.ActivityMainBinding

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val matrix = arrayOf(
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
            arrayOf("", "", "", "", "", "", "", "", ""),
        )

        var arr = ArrayList<EditText>()
        binding.apply {
            arr.addAll(
                arrayOf(
                    a00, a01, a02, a03, a04, a05, a06, a07, a08,
                    a10, a11, a12, a13, a14, a15, a16, a17, a18,
                    a20, a21, a22, a23, a24, a25, a26, a27, a28,
                    a30, a31, a32, a33, a34, a35, a36, a37, a38,
                    a40, a41, a42, a43, a44, a45, a46, a47, a48,
                    a50, a51, a52, a53, a54, a55, a56, a57, a58,
                    a60, a61, a62, a63, a64, a65, a66, a67, a68,
                    a70, a71, a72, a73, a74, a75, a76, a77, a78,
                    a80, a81, a82, a83, a84, a85, a86, a87, a88,
                )
            )
        }
        binding.solve.setOnClickListener {
            var k = 0
            for (i in 0..8) {
                for (j in 0..8) {
                    if (arr[k].text.isNotEmpty()) {
                        matrix[i][j] = arr[k].text.toString()
                    }
                    k++
                }
            }
            if (!isValidConfig(matrix)) {
                Clear(matrix, arr)
                Toast.makeText(this@MainActivity, "Invalid Sudoku", Toast.LENGTH_SHORT)
                    .show()
            } else {
                solve(matrix, 0, 0)
                k = 0
                for (i in 0..8) {
                    for (j in 0..8) {
                        arr[k].setText(matrix[i][j])
                        k++
                    }
                }
            }


        }
        binding.clear.setOnClickListener {
            Clear(matrix, arr)
        }


    }

    private fun Clear(matrix: Array<Array<String>>, arr: ArrayList<EditText>) {
        var k = 0
        for (i in 0..8) {
            for (j in 0..8) {
                matrix[i][j] = ""
                arr[k].setText("")
                k++
            }
        }
    }

    private fun solve(matrix: Array<Array<String>>, i: Int, j: Int): Boolean {
        if (i == 9) return true
        if (j == 9) return solve(matrix, i + 1, 0)
        if (matrix[i][j] != "") return solve(matrix, i, j + 1)



        for (it in 1..9) {
            if (isvalid(matrix, i, j, it)) {
                matrix[i][j] = "$it"
                if (solve(matrix, i, j + 1)) return true
                matrix[i][j] = ""
            }

        }
        return false
    }

    private fun isvalid(matrix: Array<Array<String>>, i: Int, j: Int, value: Int): Boolean {
        val row = i - (i % 3)
        val col = j - (j % 3)

        for (x in 0..8) if (matrix[x][j] == "$value") return false
        for (y in 0..8) if (matrix[i][y] == "$value") return false
        for (x in 0..2) for (y in 0..2) if (matrix[row + x][col + y] == "$value"
        ) return false
        return true

    }


    fun notInRow(matrix: Array<Array<String>>, row: Int): Boolean {

        val st = HashSet<String>()
        for (i in 0..8) {

            if (st.contains(matrix[row][i])) return false

            if (matrix[row][i] != "") st.add(matrix[row][i])
        }
        return true
    }


    fun notInCol(arr: Array<Array<String>>, col: Int): Boolean {
        val st = HashSet<String>()
        for (i in 0..8) {

            if (st.contains(arr[i][col])) return false

            if (arr[i][col] != "") st.add(arr[i][col])
        }
        return true
    }

    fun notInBox(
        arr: Array<Array<String>>,
        startRow: Int,
        startCol: Int
    ): Boolean {
        val st = HashSet<String>()
        for (row in 0..2) {
            for (col in 0..2) {
                val curr = arr[row + startRow][col + startCol]

                if (st.contains(curr)) return false

                if (curr != "") st.add(curr)
            }
        }
        return true
    }

    fun isValidConfig(arr: Array<Array<String>>): Boolean {
        val n=arr.size
        for (i in 0..n - 1) {
            for (j in 0..n - 1) {
                if (arr[i][j]!="" && !valid(arr, i, j)) return false
            }
        }
        return true
    }

    private fun valid(arr: Array<Array<String>>, row: Int, col: Int): Boolean {
        return (arr[row][col] !in "1".."9") && notInRow(arr, row) && notInCol(arr, col) &&
                notInBox(arr, row - row % 3, col - col % 3)
    }

}

Output: