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Given an unsorted array of size n. Array elements are in the range of 1 to n. One number from set {1, 2, …n} is missing and one number occurs twice in the array. Find these two numbers.

Examples: 

Input: arr[] = {3, 1, 3}
Output: Missing = 2, Repeating = 3
Explanation: In the array, 2 is missing and 3 occurs twice 

Input: arr[] = {4, 3, 6, 2, 1, 1}
Output: Missing = 5, Repeating = 1

Below are various methods to solve the problems: 

Method 1 (Use count array)
Approach: 

  • Create a temp array temp[] of size n with all initial values as 0.
  • Traverse the input array arr[], and do the following for each arr[i] 
    • if(temp[arr[i]-1] == 0), set temp[arr[i]-1] = 1;
    • if(temp[arr[i]-1] == 1) output “arr[i]” //repeating number
  • Traverse temp[] and output ‘i+1’ corresponding to the element of array temp[] having value as 0. (This is the missing number)

Note that, we use ‘arr[i]-1’ as the corresponding element to the ‘arr[i]’ in temp[] array, as indexing in an array starts from 0 to n-1 and the input array arr[] has numbers from 1 to n.

C++

#include <bits/stdc++.h>
using namespace std;
 
void printTwoElements(int arr[], int n)
{
    int temp[n] = {}; // Creating temp array of size n with
                      // initial values as 0.
    int repeatingNumber=-1;
    int missingNumber=-1;
   
    for (int i = 0; i < n; i++) {
        temp[arr[i]-1]++;
        if (temp[arr[i] - 1] > 1) {
            repeatingNumber = arr[i];
        }
    }
    for (int i = 0; i < n; i++) {
        if (temp[i] == 0) {
            missingNumber = i + 1;
            break;
        }
    }
   
    cout << "The repeating number is " << repeatingNumber<<"."
         << endl;
    cout << "The missing number is " << missingNumber<<"."
         << endl;
   
}
 
int main()
{
 
    int arr[] = { 7, 3, 4, 5, 5, 6, 2 };
    int n = sizeof(arr) / sizeof(arr[0]);
    printTwoElements(arr, n);
    return 0;
}
//  This code is contributed by vivek1208

                    

Java

import java.io.*;
 
class Main {
 
    static void printTwoElements(int[] arr, int n)
    {
        int[] temp
            = new int[n]; // Creating temp array of size n
                          // with initial values as 0.
        int repeatingNumber = -1;
        int missingNumber = -1;
 
        for (int i = 0; i < n; i++) {
            temp[arr[i] - 1]++;
            if (temp[arr[i] - 1] > 1) {
                repeatingNumber = arr[i];
            }
        }
        for (int i = 0; i < n; i++) {
            if (temp[i] == 0) {
                missingNumber = i + 1;
                break;
            }
        }
 
        System.out.println("The repeating number is "
                           + repeatingNumber + ".");
        System.out.println("The missing number is "
                           + missingNumber + ".");
    }
 
    public static void main(String[] args)
    {
        int[] arr = { 7, 3, 4, 5, 5, 6, 2 };
        int n = arr.length;
        printTwoElements(arr, n);
    }
}
 
// This code is contributed by vivek1208

                    

Python3

def printTwoElements(arr):
    n = len(arr)
    temp = [0] * # Creating temp array of size n with initial values as 0.
    repeatingNumber = -1
    missingNumber = -1
 
    for i in range(n):
        temp[arr[i] - 1] += 1
        if temp[arr[i] - 1] > 1:
            repeatingNumber = arr[i]
    for i in range(n):
        if temp[i] == 0:
            missingNumber = i + 1
            break
 
    print("The repeating number is", repeatingNumber, ".")
    print("The missing number is", missingNumber, ".")
 
 
arr = [7, 3, 4, 5, 5, 6, 2]
printTwoElements(arr)
 
# This code is contributed by vivek1208

                    

C#

using System;
 
public class Program
{
    public static void printTwoElements(int[] arr, int n)
    {
        int[] temp = new int[n]; // Creating temp array of size n with initial values as 0.
        int repeatingNumber = -1;
        int missingNumber = -1;
 
        for (int i = 0; i < n; i++)
        {
            temp[arr[i] - 1]++;
            if (temp[arr[i] - 1] > 1)
            {
                repeatingNumber = arr[i];
            }
        }
        for (int i = 0; i < n; i++)
        {
            if (temp[i] == 0)
            {
                missingNumber = i + 1;
                break;
            }
        }
 
        Console.WriteLine("The repeating number is " + repeatingNumber + ".");
        Console.WriteLine("The missing number is " + missingNumber + ".");
    }
 
    public static void Main()
    {
        int[] arr = { 7, 3, 4, 5, 5, 6, 2 };
        int n = arr.Length;
        printTwoElements(arr, n);
    }
}

                    

Javascript

function printTwoElements(arr) {
    const n = arr.length;
    const temp = Array(n).fill(0); // Creating temp array of size n with initial values as 0.
    let repeatingNumber = -1;
    let missingNumber = -1;
 
    for (let i = 0; i < n; i++) {
        temp[arr[i] - 1]++;
        if (temp[arr[i] - 1] > 1) {
            repeatingNumber = arr[i];
        }
    }
    for (let i = 0; i < n; i++) {
        if (temp[i] === 0) {
            missingNumber = i + 1;
            break;
        }
    }
 
    console.log(`The repeating number is ${repeatingNumber}.`);
    console.log(`The missing number is ${missingNumber}.`);
}
 
const arr = [7, 3, 4, 5, 5, 6, 2];
printTwoElements(arr);
 
// This code is contributed by vivek1208

                    

Output
The repeating number is 5.
The missing number is 1.


Time Complexity: O(n)
Auxiliary Space: O(n)

Method 2 (Use elements as Index and mark the visited places)
Approach: 
Traverse the array. While traversing, use the absolute value of every element as an index and make the value at this index negative to mark it visited. If something is already marked negative then this is the repeating element. To find the missing, traverse the array again and look for a positive value.

C++

// C++ program to Find the repeating
// and missing elements
 
#include <bits/stdc++.h>
using namespace std;
 
void printTwoElements(int arr[], int size)
{
    int i;
    cout << "The repeating element is ";
 
    for (i = 0; i < size; i++) {
        if (arr[abs(arr[i]) - 1] > 0)
            arr[abs(arr[i]) - 1] = -arr[abs(arr[i]) - 1];
        else
            cout << abs(arr[i]) << "\n";
    }
 
    cout << "and the missing element is ";
    for (i = 0; i < size; i++) {
        if (arr[i] > 0)
            cout << (i + 1);
    }
}
 
/* Driver code */
int main()
{
    int arr[] = { 7, 3, 4, 5, 5, 6, 2 };
    int n = sizeof(arr) / sizeof(arr[0]);
    printTwoElements(arr, n);
}
 
// This code is contributed by Shivi_Aggarwal

                    

C

// C program to Find the repeating
// and missing elements
 
#include <stdio.h>
#include <stdlib.h>
 
void printTwoElements(int arr[], int size)
{
    int i;
    printf("\nThe repeating element is ");
 
    for (i = 0; i < size; i++) {
        if (arr[abs(arr[i]) - 1] > 0)
            arr[abs(arr[i]) - 1] = -arr[abs(arr[i]) - 1];
        else
            printf(" %d ", abs(arr[i]));
    }
 
    printf("\nand the missing element is ");
    for (i = 0; i < size; i++) {
        if (arr[i] > 0)
            printf("%d", i + 1);
    }
}
 
// Driver code
int main()
{
    int arr[] = { 7, 3, 4, 5, 5, 6, 2 };
    int n = sizeof(arr) / sizeof(arr[0]);
    printTwoElements(arr, n);
    return 0;
}

                    

Java

// Java program to Find the repeating
// and missing elements
 
import java.io.*;
 
class GFG {
 
    static void printTwoElements(int arr[], int size)
    {
        int i;
        System.out.print("The repeating element is ");
 
        for (i = 0; i < size; i++) {
            int abs_val = Math.abs(arr[i]);
            if (arr[abs_val - 1] > 0)
                arr[abs_val - 1] = -arr[abs_val - 1];
            else
                System.out.println(abs_val);
        }
 
        System.out.print("and the missing element is ");
        for (i = 0; i < size; i++) {
            if (arr[i] > 0)
                System.out.println(i + 1);
        }
    }
 
    // Driver code
    public static void main(String[] args)
    {
        int arr[] = { 7, 3, 4, 5, 5, 6, 2 };
        int n = arr.length;
        printTwoElements(arr, n);
    }
}
 
// This code is contributed by Gitanjali

                    

Python3

# Python3 code to Find the repeating
# and the missing elements
 
def printTwoElements( arr, size):
    for i in range(size):
        if arr[abs(arr[i])-1] > 0:
            arr[abs(arr[i])-1] = -arr[abs(arr[i])-1]
        else:
            print("The repeating element is ", abs(arr[i]))
             
    for i in range(size):
        if arr[i]>0:
            print("and the missing element is ", i + 1)
 
# Driver program to test above function */
arr = [7, 3, 4, 5, 5, 6, 2]
n = len(arr)
printTwoElements(arr, n)
 
# This code is contributed by "Abhishek Sharma 44"

                    

C#

// C# program to Find the repeating
// and missing elements
 
using System;
 
class GFG {
    static void printTwoElements(int[] arr, int size)
    {
        int i;
        Console.Write("The repeating element is ");
 
        for (i = 0; i < size; i++) {
            int abs_val = Math.Abs(arr[i]);
            if (arr[abs_val - 1] > 0)
                arr[abs_val - 1] = -arr[abs_val - 1];
            else
                Console.WriteLine(abs_val);
        }
 
        Console.Write("and the missing element is ");
        for (i = 0; i < size; i++) {
            if (arr[i] > 0)
                Console.WriteLine(i + 1);
        }
    }
 
    // Driver program
    public static void Main()
    {
        int[] arr = { 7, 3, 4, 5, 5, 6, 2 };
        int n = arr.Length;
        printTwoElements(arr, n);
    }
}
// This code is contributed by Sam007

                    

Javascript

<script>
// Program to Find the repeating
// and missing elements
  
    function  printTwoElements(arr,size)
   {
    var i;
        document.write("The repeating element is ");
            
    for (i = 0; i < size; i++)
    {
        var abs_value = Math.abs(arr[i]);
        if (arr[abs_value - 1] > 0)
            arr[abs_value - 1] = -arr[abs_value - 1];
        else
            document.write( abs_value);
    }
              
    document.write("<br> and the missing element is ");
    for (i = 0; i < size; i++)
    {
        if (arr[i] > 0)
            document.write (i + 1);
    }
}
  
/* Driver code */
    arr = new Array ( 7, 3, 4, 5, 5, 6, 2 );
    n = arr.length;
    printTwoElements(arr, n);
     
    // This code is contributed by simranarora5sos
</script>

                    

PHP

<?php
// PHP program to Find the repeating
// and missing elements
 
function printTwoElements($arr, $size)
{
    $i;
    echo "The repeating element is", " ";
 
    for($i = 0; $i < $size; $i++)
    {
        if($arr[abs($arr[$i]) - 1] > 0)
            $arr[abs($arr[$i]) - 1] =
            - $arr[abs($arr[$i]) - 1];
        else
            echo ( abs($arr[$i]));
    }
 
    echo "\nand the missing element is ";
    for($i = 0; $i < $size; $i++)
    {
        if($arr[$i] > 0)
            echo($i + 1);
    }
}
     
    // Driver Code
    $arr = array(7, 3, 4, 5, 5, 6, 2);
    $n = count($arr);
    printTwoElements($arr, $n);
 
// This code is contributed by anuj_67.
?>

                    

Output
The repeating element is 5
and the missing element is 1

Time Complexity: O(n)
Auxiliary Space: O(1) as it is using constant variables
Thanks to Manish Mishra for suggesting this method. 

Method 3 (Make two equations)
Approach:

  • Let x be the missing and y be the repeating element.
  • Get the sum of all numbers using formula S = n(n+1)/2 – x + y
  • Get product of all numbers using formula P = 1*2*3*…*n * y / x
  • The above two steps give us two equations, we can solve the equations and get the values of x and y.

Time Complexity: O(n)
Thanks to disappearedng for suggesting this solution. 

Note: This method can cause arithmetic overflow as we calculate the product and sum of all array elements.

Method 4 (Use XOR)

Approach:

  • Let x and y be the desired output elements.
  • Calculate the XOR of all the array elements.

xor1 = arr[0]^arr[1]^arr[2]…..arr[n-1]

  • XOR the result with all numbers from 1 to n

xor1 = xor1^1^2^…..^n

  • In the result xor1, all elements would nullify each other except x and y. All the bits that are set in xor1 will be set in either x or y. So if we take any set bit (We have chosen the rightmost set bit in code) of xor1 and divide the elements of the array in two sets – one set of elements with the same bit set and another set with the same bit not set. By doing so, we will get x in one set and y in another set. Now if we do XOR of all the elements in the first set, we will get x, and by doing the same in the other set we will get y. 

Below is the implementation of the above approach: 

C++

#include <iostream>
#include <vector>
 
using namespace std;
 
int x, y;
 
// Function to find the repeating and missing elements in an
// array
void getTwoElements(vector<int>& arr, int n)
{
    int xor1 = 0; // Will hold xor of all elements and
                  // numbers from 1 to n
    int set_bit_no; // Will have only a single set bit of
                    // xor1
    x = 0;
    y = 0;
 
    xor1 = arr[0];
 
    // Get the xor of all array elements
    for (int i = 1; i < n; i++)
        xor1 = xor1 ^ arr[i];
 
    // XOR the previous result with numbers from 1 to n
    for (int i = 1; i <= n; i++)
        xor1 = xor1 ^ i;
 
    // Get the rightmost set bit in set_bit_no
    set_bit_no = xor1 & ~(xor1 - 1);
 
    // Divide elements into two sets by comparing rightmost
    // set bit of xor1 with bit at the same position in each
    // element. Also, get XORs of two sets. The two XORs are
    // the output elements.
    for (int i = 0; i < n; i++) {
        if ((arr[i] & set_bit_no) != 0)
            x = x
                ^ arr[i]; // arr[i] belongs to the first set
        else
            y = y ^ arr[i]; // arr[i] belongs to the second
                            // set
    }
 
    for (int i = 1; i <= n; i++) {
        if ((i & set_bit_no) != 0)
            x = x ^ i; // i belongs to the first set
        else
            y = y ^ i; // i belongs to the second set
    }
 
    // *x and *y hold the desired output elements
}
 
int main()
{
    vector<int> arr = { 1, 3, 4, 5, 1, 6, 2 };
    int n = arr.size();
    getTwoElements(arr, n);
    cout << "The missing element is " << x
         << " and the repeating number is " << y << endl;
    return 0;
}

                    

C

// C program to Find the repeating
// and missing elements
 
#include <stdio.h>
#include <stdlib.h>
 
/* The output of this function is stored at
   *x and *y */
void getTwoElements(int arr[], int n, int* x, int* y)
{
    /* Will hold xor of all elements and numbers
       from 1 to n */
    int xor1;
 
    /* Will have only single set bit of xor1 */
    int set_bit_no;
 
    int i;
    *x = 0;
    *y = 0;
 
    xor1 = arr[0];
 
    /* Get the xor of all array elements */
    for (i = 1; i < n; i++)
        xor1 = xor1 ^ arr[i];
 
    /* XOR the previous result with numbers
       from 1 to n*/
    for (i = 1; i <= n; i++)
        xor1 = xor1 ^ i;
 
    /* Get the rightmost set bit in set_bit_no */
    set_bit_no = xor1 & ~(xor1 - 1);
 
    /* Now divide elements in two sets by comparing
    rightmost set bit of xor1 with bit at same
    position in each element. Also, get XORs of two
    sets. The two XORs are the output elements. The
    following two for loops serve the purpose */
    for (i = 0; i < n; i++) {
        if (arr[i] & set_bit_no)
            /* arr[i] belongs to first set */
            *x = *x ^ arr[i];
 
        else
            /* arr[i] belongs to second set*/
            *y = *y ^ arr[i];
    }
    for (i = 1; i <= n; i++) {
        if (i & set_bit_no)
            /* i belongs to first set */
            *x = *x ^ i;
 
        else
            /* i belongs to second set*/
            *y = *y ^ i;
    }
 
    /* *x and *y hold the desired output elements */
}
 
/* Driver program to test above function */
int main()
{
    int arr[] = { 1, 3, 4, 5, 5, 6, 2 };
    int* x = (int*)malloc(sizeof(int));
    int* y = (int*)malloc(sizeof(int));
    int n = sizeof(arr) / sizeof(arr[0]);
 
    getTwoElements(arr, n, x, y);
    printf(" The missing element is %d"
           " and the repeating number"
           " is %d",
           *x, *y);
    getchar();
}

                    

Java

// Java program to Find the repeating
// and missing elements
 
import java.io.*;
 
class GFG {
    static int x, y;
 
    static void getTwoElements(int arr[], int n)
    {
        /* Will hold xor of all elements
       and numbers from 1 to n  */
        int xor1;
 
        /* Will have only single set bit of xor1 */
        int set_bit_no;
 
        int i;
        x = 0;
        y = 0;
 
        xor1 = arr[0];
 
        /* Get the xor of all array elements  */
        for (i = 1; i < n; i++)
            xor1 = xor1 ^ arr[i];
 
        /* XOR the previous result with numbers from
       1 to n*/
        for (i = 1; i <= n; i++)
            xor1 = xor1 ^ i;
 
        /* Get the rightmost set bit in set_bit_no */
        set_bit_no = xor1 & ~(xor1 - 1);
 
        /* Now divide elements into two sets by comparing
    rightmost set bit of xor1 with the bit at the same
    position in each element. Also, get XORs of two
    sets. The two XORs are the output elements. The
    following two for loops serve the purpose */
        for (i = 0; i < n; i++) {
            if ((arr[i] & set_bit_no) != 0)
                /* arr[i] belongs to first set */
                x = x ^ arr[i];
 
            else
                /* arr[i] belongs to second set*/
                y = y ^ arr[i];
        }
        for (i = 1; i <= n; i++) {
            if ((i & set_bit_no) != 0)
                /* i belongs to first set */
                x = x ^ i;
 
            else
                /* i belongs to second set*/
                y = y ^ i;
        }
 
        /* *x and *y hold the desired output elements */
    }
    /* Driver program to test above function */
    public static void main(String[] args)
    {
        int arr[] = { 1, 3, 4, 5, 1, 6, 2 };
 
        int n = arr.length;
        getTwoElements(arr, n);
        System.out.println(" The missing element is  "
                           + x + "and the "
                           + "repeating number is "
                           + y);
    }
}
 
// This code is contributed by Gitanjali.

                    

Python3

# Python3 program to find the repeating
# and missing elements
 
# The output of this function is stored
# at x and y
def getTwoElements(arr, n):
     
    global x, y
    x = 0
    y = 0
     
    # Will hold xor of all elements
    # and numbers from 1 to n
    xor1 = arr[0]
     
    # Get the xor of all array elements
    for i in range(1, n):
        xor1 = xor1 ^ arr[i]
         
    # XOR the previous result with numbers
    # from 1 to n
    for i in range(1, n + 1):
        xor1 = xor1 ^ i
     
    # Will have only single set bit of xor1
    set_bit_no = xor1 & ~(xor1 - 1)
     
    # Now divide elements into two
    # sets by comparing a rightmost set
    # bit of xor1 with the bit at the same
    # position in each element. Also,
    # get XORs of two sets. The two
    # XORs are the output elements.
    # The following two for loops
    # serve the purpose
    for i in range(n):
        if (arr[i] & set_bit_no) != 0:
             
            # arr[i] belongs to first set
            x = x ^ arr[i]
        else:
             
            # arr[i] belongs to second set
            y = y ^ arr[i]
             
    for i in range(1, n + 1):
        if (i & set_bit_no) != 0:
             
            # i belongs to first set
            x = x ^ i
        else:
             
            # i belongs to second set
            y = y ^ i
         
    # x and y hold the desired
    # output elements
     
# Driver code
arr = [ 1, 3, 4, 5, 5, 6, 2 ]
n = len(arr)
     
getTwoElements(arr, n)
 
print("The missing element is", x,
      "and the repeating number is", y)
     
# This code is contributed by stutipathak31jan

                    

C#

// C# program to Find the repeating
// and missing elements
 
using System;
 
class GFG {
    static int x, y;
 
    static void getTwoElements(int[] arr, int n)
    {
        /* Will hold xor of all elements
        and numbers from 1 to n */
        int xor1;
 
        /* Will have only single set bit of xor1 */
        int set_bit_no;
 
        int i;
        x = 0;
        y = 0;
 
        xor1 = arr[0];
 
        /* Get the xor of all array elements */
        for (i = 1; i < n; i++)
            xor1 = xor1 ^ arr[i];
 
        /* XOR the previous result with numbers from
        1 to n*/
        for (i = 1; i <= n; i++)
            xor1 = xor1 ^ i;
 
        /* Get the rightmost set bit in set_bit_no */
        set_bit_no = xor1 & ~(xor1 - 1);
 
        /* Now divide elements in two sets by comparing
        rightmost set bit of xor1 with bit at same
        position in each element. Also, get XORs of two
        sets. The two XORs are the output elements.The
        following two for loops serve the purpose */
        for (i = 0; i < n; i++) {
            if ((arr[i] & set_bit_no) != 0)
 
                /* arr[i] belongs to first set */
                x = x ^ arr[i];
 
            else
 
                /* arr[i] belongs to second set*/
                y = y ^ arr[i];
        }
        for (i = 1; i <= n; i++) {
            if ((i & set_bit_no) != 0)
 
                /* i belongs to first set */
                x = x ^ i;
 
            else
 
                /* i belongs to second set*/
                y = y ^ i;
        }
 
        /* *x and *y hold the desired output elements */
    }
 
    // Driver program
    public static void Main()
    {
        int[] arr = { 1, 3, 4, 5, 1, 6, 2 };
 
        int n = arr.Length;
        getTwoElements(arr, n);
        Console.Write(" The missing element is "
                      + x + "and the "
                      + "repeating number is "
                      + y);
    }
}
 
// This code is contributed by Sam007

                    

Javascript

// Javascript program to Find the repeating
// and missing elements
let x, y;
 
    function getTwoElements(arr, n)
    {
        /* Will hold xor of all elements
       and numbers from 1 to n  */
        let xor1;
 
        /* Will have only single set bit of xor1 */
        let set_bit_no;
 
        let i;
        x = 0;
        y = 0;
 
        xor1 = arr[0];
 
        /* Get the xor of all array elements  */
        for (i = 1; i < n; i++)
            xor1 = xor1 ^ arr[i];
 
        /* XOR the previous result with numbers from
       1 to n*/
        for (i = 1; i <= n; i++)
            xor1 = xor1 ^ i;
 
        /* Get the rightmost set bit in set_bit_no */
        set_bit_no = xor1 & ~(xor1 - 1);
 
        /* Now divide elements into two sets by comparing
    rightmost set bit of xor1 with the bit at the same
    position in each element. Also, get XORs of two
    sets. The two XORs are the output elements. The
    following two for loops serve the purpose */
        for (i = 0; i < n; i++) {
            if ((arr[i] & set_bit_no) != 0)
                /* arr[i] belongs to first set */
                x = x ^ arr[i];
 
            else
                /* arr[i] belongs to second set*/
                y = y ^ arr[i];
        }
        for (i = 1; i <= n; i++) {
            if ((i & set_bit_no) != 0)
                /* i belongs to first set */
                x = x ^ i;
 
            else
                /* i belongs to second set*/
                y = y ^ i;
        }
 
        /* *x and *y hold the desired output elements */
    }
    
        let arr = [ 1, 3, 4, 5, 1, 6, 2 ];
 
        let n = arr.length;
        getTwoElements(arr, n);
        console.log(" The missing element is  "
                           + x + " and the "
                           + "repeating number is "
                           + y);
     
    // This code is contributed by garg28harsh.

                    

PHP

<?php
// PHP program to Find the repeating
// and missing elements
 
function getTwoElements(&$arr, $n)
{
 
    /* Will hold xor of all elements
    and numbers from 1 to n */
    $xor1;
 
    /* Will have only single set bit of xor1 */
    $set_bit_no;
 
    $i;
    $x = 0;
    $y = 0;
 
    $xor1 = $arr[0];
 
    /* Get the xor of all array elements */
    for ($i = 1; $i < $n; $i++)
        $xor1 = $xor1 ^ $arr[$i];
 
    /* XOR the previous result with numbers
    from 1 to n*/
    for ($i = 1; $i <= $n; $i++)
        $xor1 = $xor1 ^ $i;
 
    /* Get the rightmost set bit in set_bit_no */
    $set_bit_no = $xor1 & ~($xor1 - 1);
 
    /* Now divide elements in two sets by comparing
    rightmost set bit of xor1 with bit at same
    position in each element. Also, get XORs of two
    sets. The two XORs are the output elements.The
    following two for loops serve the purpose */
    for ($i = 0; $i < $n; $i++)
    {
        if (($arr[$i] & $set_bit_no) != 0)
         
            /* arr[i] belongs to first set */
            $x = $x ^ $arr[$i];
 
        else
         
            /* arr[i] belongs to second set*/
            $y = $y ^ $arr[$i];
    }
     
    for ($i = 1; $i <= $n; $i++)
    {
        if (($i & $set_bit_no) != 0)
         
            /* i belongs to first set */
            $x = $x ^ $i;
 
        else
         
            /* i belongs to second set*/
            $y = $y ^ $i;
    }
 
    /* *x and *y hold the desired output elements */
    echo("The missing element is " . $x .
         " and the repeating number is " . $y);
}
 
// Driver Code
$arr = array( 1, 3, 4, 5, 1, 6, 2 );
$n = sizeof($arr);
getTwoElements($arr, $n);
 
// This code is contributed by Code_Mech

                    

Output
 The missing element is 7 and the repeating number is 5

Time Complexity: O(n)
Auxiliary Space: O(1) as it is using constant space if the input array is excluded
This method doesn’t cause overflow, but it doesn’t tell which one occurs twice and which one is missing. We can add one more step that checks which one is missing and which one is repeating. This can be easily done in O(n) time.

Method 5 (Use a Map)
Approach: 
This method involves creating a Hashtable with the help of Map. In this, the elements are mapped to their natural index. In this process, if an element is mapped twice, then it is the repeating element. And if an element’s mapping is not there, then it is the missing element.

Below is the implementation of the above approach: 

C++

// C++ program to find the repeating
// and missing elements using Maps
#include <iostream>
#include <unordered_map>
using namespace std;
 
int main()
{
    int arr[] = { 4, 3, 6, 2, 1, 1 };
    int n = 6;
     
    unordered_map<int, bool> numberMap;
     
    for(int i : arr)
    {
        if (numberMap.find(i) ==
            numberMap.end())
        {
            numberMap[i] = true;
        }
        else
        {
            cout << "Repeating = " << i;
              break;
        }
    }
    cout << endl;
     
    for(int i = 1; i <= n; i++)
    {
        if (numberMap.find(i) ==
            numberMap.end())
        {
            cout << "Missing = " << i;
              break;
        }
    }
    return 0;
}
 
// This code is contributed by RohitOberoi

                    

Java

// Java program to find the
// repeating and missing elements
// using Maps
 
import java.util.*;
 
public class Test1 {
 
    public static void main(String[] args)
    {
 
        int[] arr = { 4, 3, 6, 2, 1, 1 };
 
        Map<Integer, Boolean> numberMap
            = new HashMap<>();
 
        int max = arr.length;
 
        for (Integer i : arr) {
 
            if (numberMap.get(i) == null) {
                numberMap.put(i, true);
            }
            else {
                System.out.println("Repeating = " + i);
            }
        }
        for (int i = 1; i <= max; i++) {
            if (numberMap.get(i) == null) {
                System.out.println("Missing = " + i);
            }
        }
    }
}

                    

Python3

# Python3 program to find the
# repeating and missing elements
# using Maps
def main():
     
    arr = [ 4, 3, 6, 2, 1, 1 ]
     
    numberMap = {}
     
    max = len(arr)
    for i in arr:
        if not i in numberMap:
            numberMap[i] = True
             
        else:
            print("Repeating =", i)
     
    for i in range(1, max + 1):
        if not i in numberMap:
            print("Missing =", i)
main()
 
# This code is contributed by stutipathak31jan

                    

C#

// C# program to find the
// repeating and missing elements
// using Maps
using System;
using System.Collections.Generic;
 
class GFG
{
    public static void Main(String[] args)
    {
        int[] arr = { 4, 3, 6, 2, 1, 1 };
 
        Dictionary<int, Boolean> numberMap =
                   new Dictionary<int, Boolean>();
 
        int max = arr.Length;
 
        foreach (int i in arr)
        {
            if (!numberMap.ContainsKey(i))
            {
                numberMap.Add(i, true);
            }
            else
            {
                Console.WriteLine("Repeating = " + i);
            }
        }
        for (int i = 1; i <= max; i++)
        {
            if (!numberMap.ContainsKey(i))
            {
                Console.WriteLine("Missing = " + i);
            }
        }
    }
}
 
// This code is contributed by PrinciRaj1992

                    

Javascript

<script>
 
// JavaScript program to find the repeating
// and missing elements using Maps
 
// driver program
let arr = [ 4, 3, 6, 2, 1, 1 ];
let n = 6;
     
let numberMap = new Map();
     
for(let i of arr)
{
    if (numberMap.has(i) == false) {
        numberMap.set(i,true);
    }
    else
    {
       document.write("Repeating = ",i);
    }
}
   document.write("</br>");
     
for(let i = 1; i <= n; i++)
{
    if (numberMap.has(i) == false) {
       document.write("Missing = " ,i);
    }
}
 
// This code is contributed by Shinjanpatra
</script>

                    

Output
Repeating = 1
Missing = 5

Time Complexity: O(N)
Auxiliary Space: O(N)

Method 6 (Make two equations using sum and sum of squares)
Approach:

  • Let x be the missing and y be the repeating element.
  • Let N is the size of the array.
  • Get the sum of all numbers using the formula S = N(N+1)/2
  • Get the sum of square of all numbers using formula Sum_Sq = N(N+1)(2N+1)/6
  • Iterate through a loop from i=1….N
  • S -= A[i]
  • Sum_Sq -= (A[i]*A[i])
  • It will give two equations 
    x-y = S – (1) 
    x^2 – y^2 = Sum_sq 
    x+ y = (Sum_sq/S) – (2) 

C++

#include <bits/stdc++.h>
 
using namespace std;
 
vector<int>repeatedNumber(const vector<int> &A) {
    long long int len = A.size();
    long long int Sum_N = (len * (len+1) ) /2, Sum_NSq = (len * (len +1) *(2*len +1) )/6;
    long long int missingNumber=0, repeating=0;
     
    for(int i=0;i<A.size(); i++){
       Sum_N -= (long long int)A[i];
       Sum_NSq -= (long long int)A[i]*(long long int)A[i];
    }
     
    missingNumber = (Sum_N + Sum_NSq/Sum_N)/2;
    repeating= missingNumber - Sum_N;
    vector <int> ans;
    ans.push_back(repeating);
    ans.push_back(missingNumber);
    return ans;
     
}
 
 
int main(void){
        std::vector<int> v = {4, 3, 6, 2, 1, 6,7};
    vector<int> res = repeatedNumber(v);
    for(int x: res){
        cout<< x<<"  ";
    }
    cout<<endl;
}

                    

Java

import java.util.*;
import java.math.BigInteger;
class GFG
{
    static Vector<Integer> repeatedNumber(int[] a)
    {
        
        BigInteger n=BigInteger.valueOf(a.length);
   
        BigInteger s=BigInteger.valueOf(0);
        BigInteger ss=BigInteger.valueOf(0);
 
        for(int x : a)
        {
            s=  s.add(BigInteger.valueOf(x));
            ss= ss.add(BigInteger.valueOf(x).multiply(BigInteger.valueOf(x)));
        }
 
        BigInteger as= n.multiply(n.add(BigInteger.valueOf(1))).divide(BigInteger.valueOf(2));
        BigInteger ass= as.multiply(BigInteger.valueOf(2).multiply(n).add(BigInteger.valueOf(1))).divide(BigInteger.valueOf(3));
 
        BigInteger sub=as.subtract(s);
        BigInteger add=(ass.subtract(ss)).divide(sub);
        //(ass-ss)/sub;
 
        int b = sub.add(add).divide(BigInteger.valueOf(2)).intValue();
        //(sub+add)/2;
        int A = BigInteger.valueOf(b).subtract(sub).intValue();
        Vector<Integer> ans = new Vector<>();
        ans.add(A);
        ans.add(b);
        return ans;
    }
 
    // Driver Code
    public static void main(String[] args)
    {
        int[] v = { 4, 3, 6, 2, 1, 6, 7 };
        Vector<Integer> res = repeatedNumber(v);
        for (int x : res)
        {
            System.out.print(x + " ");
        }
    }
}
 
// This code is contributed by Rajput-Ji

                    

Python3

def repeatedNumber(A):
     
    length = len(A)
    Sum_N = (length * (length + 1)) // 2
    Sum_NSq = ((length * (length + 1) *
                     (2 * length + 1)) // 6)
     
    missingNumber, repeating = 0, 0
     
    for i in range(len(A)):
        Sum_N -= A[i]
        Sum_NSq -= A[i] * A[i]
         
    missingNumber = (Sum_N + Sum_NSq //
                             Sum_N) // 2
    repeating = missingNumber - Sum_N
     
    ans = []
    ans.append(repeating)
    ans.append(missingNumber)
     
    return ans
 
# Driver code
v = [ 4, 3, 6, 2, 1, 6, 7 ]
res = repeatedNumber(v)
 
for i in res:
    print(i, end = " ")
 
# This code is contributed by stutipathak31jan

                    

C#

using System;
using System.Collections.Generic;
 
class GFG
{
    static List<int> repeatedNumber(int[] A)
    {
        int len = A.Length;
        int Sum_N = (len * (len + 1)) / 2;
        int Sum_NSq = (len * (len + 1) *
                        (2 * len + 1)) / 6;
        int missingNumber = 0, repeating = 0;
 
        for (int i = 0; i < A.Length; i++)
        {
            Sum_N -= A[i];
            Sum_NSq -= A[i] * A[i];
        }
 
        missingNumber = (Sum_N + Sum_NSq /
                                 Sum_N) / 2;
        repeating = missingNumber - Sum_N;
        List<int> ans = new List<int>();
        ans.Add(repeating);
        ans.Add(missingNumber);
        return ans;
    }
 
    // Driver Code
    public static void Main(String[] args)
    {
        int[] v = { 4, 3, 6, 2, 1, 6, 7 };
        List<int> res = repeatedNumber(v);
        foreach (int x in res)
        {
            Console.Write(x + " ");
        }
    }
}
 
// This code is contributed by PrinciRaj1992

                    

Javascript

<script>
 
function repeatedNumber(A){
     
    let length = A.length
    let Sum_N = Math.floor((length * (length + 1)) / 2)
    let Sum_NSq = Math.floor((length * (length + 1) * (2 * length + 1))/6)
 
    let missingNumber = 0
    let repeating = 0
 
    for(let i=0;i<A.length;i++){
        Sum_N -= A[i]
        Sum_NSq -= A[i] * A[i]
    }
     
    missingNumber = Math.floor(Math.floor(Sum_N + Sum_NSq / Sum_N) / 2)
    repeating = missingNumber - Sum_N
 
    let ans = []
    ans.push(repeating)
    ans.push(missingNumber)
 
return ans
 
}
 
// Driver code
let v = [ 4, 3, 6, 2, 1, 6, 7 ]
let res = repeatedNumber(v)
 
for(let i of res)
    document.write(i," ")
 
// This code is contributed by shinjanpatra
 
</script>

                    

Output
6  5  


Time Complexity: O(n) 
Auxiliary Space: O(1)



Last Updated : 13 Sep, 2023
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