# Rearrange an array such that arr[i] = i

Given an array of elements of length N, ranging from 0 to N – 1. All elements may not be present in the array. If the element is not present then there will be -1 present in the array. Rearrange the array such that A[i] = i and if i is not present, display -1 at that place.
Examples:

```Input : arr = {-1, -1, 6, 1, 9, 3, 2, -1, 4, -1}
Output : [-1, 1, 2, 3, 4, -1, 6, -1, -1, 9]

Input : arr = {19, 7, 0, 3, 18, 15, 12, 6, 1, 8,
11, 10, 9, 5, 13, 16, 2, 14, 17, 4}
Output : [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
11, 12, 13, 14, 15, 16, 17, 18, 19]

```

Approach(Naive Approach):

1. Nav­i­gate the numbers from 0 to n-1.
2. Now navigate through array.
3. If (i==a[j]) , then replace the element at i position with a[j] position.
4. If there is any element in which -1 is used instead of the number then it will be replaced automatically.
5. Now, iterate through the array and check if (a[i]!=i) , if it s true then replace a[i] with -1.

Below is the implementation for the above approach:

## C++

 `// CPP program for above approach` `#include` `using` `namespace` `std;`   `// Function to tranform the array` `void` `fix(``int` `ar[], ``int` `n)` `{` `    ``int` `i,j,temp;` `    `  `    ``// Iterate over the array` `    ``for``(i = 0; i < n; i++)` `    ``{` `        ``for``(j = 0; j < n; j++)` `        ``{` `            `  `            ``// Checf is any ar[j]` `            ``// exists such that ` `            ``// ar[j] is equal to i` `            ``if``(ar[j] == i)` `            ``{` `                ``temp = ar[j];` `                ``ar[j] = ar[i];` `                ``ar[i] = temp;` `                ``break``;` `            ``}            ` `        ``}` `    ``}` `  `  `    ``// Iterate over array` `    ``for``(i = 0; i < n; i++)` `    ``{` `        ``// If not present` `        ``if``(ar[i] != i)` `        ``{` `            ``ar[i] =- 1;` `        ``}            ` `    ``}` `  `  `    ``// Print the output` `    ``cout << ``"Array after Rearranging"` `<< endl;` `    ``for``(i = 0; i < n; i++)` `    ``{` `        ``cout << ar[i] << ``" "``;` `    ``}` `}`   `// Driver Code` `int` `main()` `{` `    ``int` `n, ar[]={-1, -1, 6, 1, 9, ` `                          ``3, 2, -1, 4, -1};` `       ``n = ``sizeof``(ar) / ``sizeof``(ar);` `    `  `    ``// Function Call` `    ``fix(ar, n);` `}`   `// Code BY Tanmay Anand`

Output

```Array after Rearranging
-1 1 2 3 4 -1 6 -1 -1 9
```

Time Complexity: O(n2)

Another Approach:
1. Nav­i­gate the array.
2. Check if a[i] = -1, if yes then ignore it.
3. If a[i] != -1, Check if element a[i] is at its cor­rect posi­tion (i=A[i]). If yes then ignore it.
4. If a[i] != -1 and ele­ment a[i] is not at its cor­rect posi­tion (i!=A[i]) then place it to its correct posi­tion, but there are two conditions:

• Either A[i] is vacate, means A[i] = -1, then just put A[i] = i .
• OR A[i] is not vacate, means A[i] = x, then int y=x put A[i] = i. Now, we need to place y to its cor­rect place, so repeat from step 3.

Below is the implementation for the above approach:

## C++

 `// CPP program for rearrange an` `// array such that arr[i] = i.` `#include `   `using` `namespace` `std;`   `// Function to rearrange an array` `// such that arr[i] = i.` `int` `fix(``int` `A[], ``int` `len)` `{` `    ``for` `(``int` `i = 0; i < len; i++) ` `    ``{` `        ``if` `(A[i] != -1 && A[i] != i) ` `        ``{` `            ``int` `x = A[i];`   `            ``// check if desired place` `            ``// is not vacate` `            ``while` `(A[x] != -1 && A[x] != x)` `            ``{` `                ``// store the value from` `                ``// desired place` `                ``int` `y = A[x];`   `                ``// place the x to its correct` `                ``// position` `                ``A[x] = x;`   `                ``// now y will become x, now` `                ``// search the place for x` `                ``x = y;` `            ``}`   `            ``// place the x to its correct` `            ``// position` `            ``A[x] = x;`   `            ``// check if while loop hasn't` `            ``// set the correct value at A[i]` `            ``if` `(A[i] != i) ` `            ``{`   `                ``// if not then put -1 at` `                ``// the vacated place` `                ``A[i] = -1;` `            ``}` `        ``}` `    ``}` `}`   `// Driver function.` `int` `main()` `{` `    ``int` `A[] = { -1, -1, 6, 1, 9,` `                ``3, 2, -1, 4, -1 };`   `    ``int` `len = ``sizeof``(A) / ``sizeof``(A);` `    ``fix(A, len);`   `    ``for` `(``int` `i = 0; i < len; i++)` `        ``cout << A[i] << ``" "``;` `}`   `// This code is contributed by Smitha Dinesh Semwal`

## Java

 `// Java program for rearrange an ` `// array such that arr[i] = i.` `import` `java.util.*;` `import` `java.lang.*;`   `public` `class` `GfG {`   `    ``// Function to rearrange an array` `    ``// such that arr[i] = i.` `    ``public` `static` `int``[] fix(``int``[] A)` `    ``{` `        ``for` `(``int` `i = ``0``; i < A.length; i++) ` `        ``{` `            ``if` `(A[i] != -``1` `&& A[i] != i)` `            ``{` `                ``int` `x = A[i];`   `                ``// check if desired place` `                ``// is not vacate` `                ``while` `(A[x] != -``1` `&& A[x] != x) ` `                ``{`   `                    ``// store the value from` `                    ``// desired place` `                    ``int` `y = A[x];`   `                    ``// place the x to its correct` `                    ``// position` `                    ``A[x] = x;`   `                    ``// now y will become x, now` `                    ``// search the place for x` `                    ``x = y;` `                ``}`   `                ``// place the x to its correct` `                ``// position` `                ``A[x] = x;`   `                ``// check if while loop hasn't ` `                ``// set the correct value at A[i]` `                ``if` `(A[i] != i)` `                ``{`   `                    ``// if not then put -1 at` `                    ``// the vacated place` `                    ``A[i] = -``1``;` `                ``}` `            ``}` `        ``}` `        ``return` `A;` `    ``}`   `    ``// Driver function.` `    ``public` `static` `void` `main(String[] args)` `    ``{` `        ``int` `A[] = {-``1``, -``1``, ``6``, ``1``, ` `                    ``9``, ``3``, ``2``, -``1``, ``4``,-``1``};` `        ``System.out.println(Arrays.toString(fix(A)));` `    ``}` `}`

## Python3

 `# Python3 program for rearrange an` `# array such that arr[i] = i.`   `# Function to rearrange an array` `# such that arr[i] = i.` `def` `fix( A, ``len``):` `    `  `    ``for` `i ``in` `range``(``0``, ``len``): ` `        `  `        ``if` `(A[i] !``=` `-``1` `and` `A[i] !``=` `i):` `            ``x ``=` `A[i];`   `            ``# check if desired place` `            ``# is not vacate` `            ``while` `(A[x] !``=` `-``1` `and` `A[x] !``=` `x):` `                ``#store the value from` `                ``# desired place` `                ``y ``=` `A[x]`   `                ``# place the x to its correct` `                ``# position` `                ``A[x] ``=` `x`   `                ``# now y will become x, now` `                ``# search the place for x` `                ``x ``=` `y` `            `  `            ``# place the x to its correct` `            ``# position` `            ``A[x] ``=` `x;`   `            ``# check if while loop hasn't` `            ``# set the correct value at A[i]` `            ``if` `(A[i] !``=` `i) :` `                `  `                ``# if not then put -1 at` `                ``# the vacated place` `                ``A[i] ``=` `-``1``;`   `# Driver function.` `A ``=` `[ ``-``1``, ``-``1``, ``6``, ``1``, ``9``, ``3``, ``2``, ``-``1``, ``4``, ``-``1` `]`   `fix(A, ``len``(A))`   `for` `i ``in` `range``(``0``, ``len``(A)):` `    ``print` `(A[i],end ``=` `' '``)` `    `  `# This code is contributed by Saloni1297`

## C#

 `// C# program for rearrange ` `// an array such that ` `// arr[i] = i.` `using` `System;`   `class` `GfG ` `{` `// Function to rearrange an ` `// array such that arr[i] = i.` `public` `static` `int``[] fix(``int``[] A)` `{` `    ``for` `(``int` `i = 0; ` `             ``i < A.Length; i++) ` `    ``{` `        ``if` `(A[i] != -1 && ` `            ``A[i] != i)` `        ``{` `            ``int` `x = A[i];`   `            ``// check if desired ` `            ``// place is not vacate` `            ``while` `(A[x] != -1 && ` `                   ``A[x] != x) ` `            ``{`   `                ``// store the value ` `                ``// from desired place` `                ``int` `y = A[x];`   `                ``// place the x to its` `                ``// correct position` `                ``A[x] = x;`   `                ``// now y will become x, ` `                ``// now search the place ` `                ``// for x` `                ``x = y;` `            ``}`   `            ``// place the x to its ` `            ``// correct position` `            ``A[x] = x;`   `            ``// check if while loop ` `            ``// hasn't set the correct ` `            ``// value at A[i]` `            ``if` `(A[i] != i)` `            ``{`   `                ``// if not then put -1 ` `                ``// at the vacated place` `                ``A[i] = -1;` `            ``}` `        ``}` `    ``}` `    ``return` `A;` `}`   `// Driver Code` `static` `void` `Main()` `{` `    ``int` `[]A = ``new` `int``[]{-1, -1, 6, 1, 9, ` `                         ``3, 2, -1, 4,-1};` `    ``Console.WriteLine(``string``.Join(``","``,` `                                 ``fix(A)));` `}` `}`   `// This code is contributed by ` `// Manish Shaw(manishshaw1)`

## PHP

 ``

Output

```-1 1 2 3 4 -1 6 -1 -1 9
```

Another Approach (Using HashSet) :
1) Store all the numbers present in the array into a HashSet
2) Iterate through the length of the array, if the corresponding position element is present in the HashSet, then set A[i] = i, else A[i] = -1
Below is the implementation of the above approach.

## Java

 `// Java program for rearrange an ` `// array such that arr[i] = i.` `import` `java.util.*;` `import` `java.lang.*;`   `class` `GfG {`   `    ``// Function to rearrange an array` `    ``// such that arr[i] = i.` `    ``public` `static` `int``[] fix(``int``[] A)` `    ``{` `        ``Set s = ``new` `HashSet();`   `        ``// Storing all the values in the HashSet` `        ``for``(``int` `i = ``0``; i < A.length; i++)` `        ``{` `           ``s.add(A[i]);` `        ``}`   `        ``for``(``int` `i = ``0``; i < A.length; i++)` `        ``{` `           ``if``(s.contains(i))` `             ``A[i] = i;` `           ``else` `             ``A[i] = -``1``;` `        ``}`   `      ``return` `A;` `}`   `    ``// Driver function.` `    ``public` `static` `void` `main(String[] args)` `    ``{` `        ``int` `A[] = {-``1``, -``1``, ``6``, ``1``, ``9``,` `                    ``3``, ``2``, -``1``, ``4``,-``1``};` `                    `  `        ``// Function calling` `        ``System.out.println(Arrays.toString(fix(A)));` `    ``}` `}`

## Python3

 `# Python3 program for rearrange an ` `# array such that arr[i] = i.`   `# Function to rearrange an array` `# such that arr[i] = i.` `def` `fix(A):` `    ``s ``=` `set``()` `    `  `    ``# Storing all the values in the Set` `    ``for` `i ``in` `range``(``len``(A)):` `        ``s.add(A[i])`   `    ``for` `i ``in` `range``(``len``(A)):` `      `  `        ``# check for item if present in set` `        ``if` `i ``in` `s:` `            ``A[i] ``=` `i` `        ``else``:` `            ``A[i] ``=` `-``1` `    ``return` `A` `    `  `# Driver code` `if` `__name__ ``=``=` `"__main__"``:` `    ``A ``=` `[``-``1``, ``-``1``, ``6``, ``1``, ``9``,` `          ``3``, ``2``, ``-``1``, ``4``,``-``1``]` `    ``print``(fix(A))`   `# This code is contributed by Chitranayal`

## C#

 `// C# program for rearrange an ` `// array such that arr[i] = i.` `using` `System;`   `class` `GfG` `{`   `    ``// Function to rearrange ` `    ``// an array such that` `    ``// arr[i] = i.` `    ``static` `int``[] fix(``int` `[]A)` `    ``{` `        ``for` `(``int` `i = 0; ` `                 ``i < A.Length; i++) ` `        ``{` `            ``if` `(A[i] != -1 && ` `                ``A[i] != i)` `            ``{` `                ``int` `x = A[i];`   `                ``// check if desired place` `                ``// is not vacate` `                ``while` `(A[x] != -1 && ` `                       ``A[x] != x) ` `                ``{`   `                    ``// store the value from` `                    ``// desired place` `                    ``int` `y = A[x];`   `                    ``// place the x to its ` `                    ``// correct position` `                    ``A[x] = x;`   `                    ``// now y will become x, now` `                    ``// search the place for x` `                    ``x = y;` `                ``}`   `                ``// place the x to its ` `                ``// correct position` `                ``A[x] = x;`   `                ``// check if while loop hasn't ` `                ``// set the correct value at A[i]` `                ``if` `(A[i] != i)` `                ``{ ` `                    ``// if not then put -1 at` `                    ``// the vacated place` `                    ``A[i] = -1;` `                ``}` `            ``}` `        ``}` `        ``return` `A;` `    ``} ` `    `  `    ``// Driver Code` `    ``static` `void` `Main()` `    ``{` `        ``int` `[]A = ``new` `int``[]{-1, -1, 6, 1, 9, ` `                             ``3, 2, -1, 4,-1};` `        ``Console.Write(``string``.Join(``","``, fix(A)));` `    ``}` `}`   `// This code is contributed by ` `// Manish Shaw(manishshaw1)`

Output

```[-1, 1, 2, 3, 4, -1, 6, -1, -1, 9]

```

Another Approach (Swap elements in Array) :
1) Iterate through elements in array
2) If arr[i] >= 0 && arr[i] != i, put arr[i] at i ( swap arr[i] with arr[arr[i]])
Below is the implementation of above approach.

## C++

 `// C++ program for rearrange an ` `// array such that arr[i] = i.` `#include`   `int` `main() ` `{` `    ``int` `arr[] = {-1, -1, 6, 1, 9, 3, 2, -1, 4, -1};` `    ``int` `n = ``sizeof``(arr)/``sizeof``(arr);` `    ``for` `(``int` `i = 0; i < n;) ` `    ``{` `        ``if` `(arr[i] >= 0 && arr[i] != i) ` `        ``arr[arr[i]] = (arr[arr[i]] + arr[i]) - ` `                        ``(arr[i] = arr[arr[i]]);` `        ``else` `            ``i++;` `    ``}` `    ``for``(``int` `i = 0; i < n; i++)` `        ``printf``(``"%d "``,arr[i]);` `        ``return` `0;` `}`   `// This Code is Contributed by Adarsh_Verma `

## Java

 `// Java program for rearrange an ` `// array such that arr[i] = i.` `import` `java.util.Arrays;`   `class` `Program {`   `    ``public` `static` `void` `main(String[] args) {` `        ``int``[] arr = {-``1``, -``1``, ``6``, ``1``, ``9``, ``3``, ``2``, -``1``, ``4``, -``1``};` `        ``for` `(``int` `i = ``0``; i < arr.length;) {` `            ``if` `(arr[i] >= ``0` `&& arr[i] != i) {` `                ``int` `ele = arr[arr[i]];` `                ``arr[arr[i]] = arr[i];` `                ``arr[i] = ele;` `            ``} ``else` `{` `                ``i++;` `            ``}` `        ``}` `        ``System.out.println(Arrays.toString(arr));` `    ``}` `}`   ` `  `/* This Java code is contributed by PrinciRaj1992*/`

## Python3

 `# Python3 program for rearrange an ` `# array such that arr[i] = i.` `if` `__name__ ``=``=` `"__main__"``:` `  `  `    ``arr ``=` `[``-``1``, ``-``1``, ``6``, ``1``, ``9``, ` `            ``3``, ``2``, ``-``1``, ``4``, ``-``1``]` `    ``n ``=` `len``(arr)` `    ``i ``=` `0` `    ``while` `i < n:` `   `  `        ``if` `(arr[i] >``=` `0` `and` `arr[i] !``=` `i):` `            ``(arr[arr[i]] , ` `             ``arr[i]) ``=` `(arr[i] , ` `                        ``arr[arr[i]])` `        ``else``:` `            ``i ``+``=` `1` `    `  `    ``for` `i ``in` `range` `(n):` `        ``print``(arr[i], end``=` `" "``)` `       `  `# This code is contributed by Chitranayal`

## C#

 `// C# program for rearrange an ` `// array such that arr[i] = i.` `using` `System;`   `namespace` `GFG` `{` `    ``class` `Program` `    ``{` `        ``static` `void` `Main(``string``[] args)` `        ``{` `            ``int``[] arr = {-1, -1, 6, 1, 9, 3, 2, -1, 4, -1};` `            ``for``(``int` `i = 0; i = 0 && arr[i] != i)` `                ``{` `                    ``int` `ele = arr[arr[i]];` `                    ``arr[arr[i]] = arr[i];` `                    ``arr[i] = ele;` `                ``} ``else` `{` `                    ``i++;` `                ``}` `            ``}` `            ``Console.WriteLine(String.Join(``","``, arr));` `        ``}` `    ``}` `}` `// This code is contributed by ` `// Venkata VardhiReddy(venkata)`

Output

```-1 1 2 3 4 -1 6 -1 -1 9
```

Time Complexity: O(n)

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