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Find index of an extra element present in one sorted array

  • Difficulty Level : Easy
  • Last Updated : 25 Jun, 2021

Given two sorted arrays. There is only 1 difference between the arrays. The first array has one element extra added in between. Find the index of the extra element.

Examples: 

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Input: {2, 4, 6, 8, 9, 10, 12};
       {2, 4, 6, 8, 10, 12};
Output: 4
Explanation: The first array has an extra element 9.
The extra element is present at index 4.

Input: {3, 5, 7, 9, 11, 13}
        {3, 5, 7, 11, 13}
Output: 3
Explanation: The first array has an extra element 9.
The extra element is present at index 3.

Method 1: This includes the basic approach to solve this particular problem. 

Approach: The basic method is to iterate through the whole second array and check element by element if they are different. As the array is sorted, checking the adjacent position of two arrays should be similar until and unless the missing element is found. 



Algorithm: 

  1. Traverse through the array from start to end.
  2. Check if the element at i’th element of the two arrays is similar or not.
  3. If the elements are not similar then print the index and break

Implementation: 

C++




// C++ program to find an extra
// element present in arr1[]
#include <iostream>
using namespace std;
 
// Returns index of extra element
// in arr1[]. n is size of arr2[].
// Size of arr1[] is n-1.
int findExtra(int arr1[],
              int arr2[], int n)
{
for (int i = 0; i < n; i++)
    if (arr1[i] != arr2[i])
        return i;
 
return n;
}
 
// Driver code
int main()
{
    int arr1[] = {2, 4, 6, 8,
                  10, 12, 13};
    int arr2[] = {2, 4, 6,
                  8, 10, 12};
    int n = sizeof(arr2) / sizeof(arr2[0]);
 
    // Solve is passed both arrays
    cout << findExtra(arr1, arr2, n);
    return 0;
}

Java




// Java program to find an extra
// element present in arr1[]
class GFG
{
 
    // Returns index of extra element
    // in arr1[]. n is size of arr2[].
    // Size of arr1[] is n-1.
    static int findExtra(int arr1[],
                         int arr2[], int n)
    {
    for (int i = 0; i < n; i++)
        if (arr1[i] != arr2[i])
            return i;
     
    return n;
    }
     
    // Driver Code
    public static void main (String[] args)
    {
        int arr1[] = {2, 4, 6, 8,
                      10, 12, 13};
        int arr2[] = {2, 4, 6,
                      8, 10, 12};
        int n = arr2.length;
     
        // Solve is passed both arrays
        System.out.println(findExtra(arr1,
                                     arr2, n));
    }
}
 
// This code is contributed by Harsh Agarwal

Python3




# Python 3 program to find an
# extra element present in arr1[]
 
 
# Returns index of extra .
# element in arr1[] n is
# size of arr2[]. Size of
# arr1[] is n-1.
def findExtra(arr1, arr2, n) :
    for i in range(0, n) :
        if (arr1[i] != arr2[i]) :
            return i
 
    return n
 
 
# Driver code
arr1 = [2, 4, 6, 810, 12, 13]
arr2 = [2, 4, 6, 8, 10, 12]
n = len(arr2)
 
# Solve is passed both arrays
print(findExtra(arr1, arr2, n))
 
# This code is contributed
# by Nikita Tiwari.

C#




// C# program to find an extra
// element present in arr1[]
using System;
 
class GfG
{
     
    // Returns index of extra
    // element in arr1[]. n is
    // size of arr2[]. Size of
    // arr1[] is n-1.
    static int findExtra(int []arr1,
                         int []arr2, int n)
    {
        for (int i = 0; i < n; i++)
            if (arr1[i] != arr2[i])
                return i;
         
        return n;
    }
     
    // Driver code
    public static void Main ()
    {
        int []arr1 = {2, 4, 6, 8,
                      10, 12, 13};
        int []arr2 = {2, 4, 6,
                      8, 10, 12};
        int n = arr2.Length;
     
        // Solve is passed both arrays
        Console.Write(findExtra(arr1, arr2, n));
    }
}
 
// This code is contributed by parashar.

PHP




<?php
// PHP program to find an extra
// element present in arr1[]
 
// Returns index of extra element
// in arr1[]. n is size of arr2[].
// Size of arr1[] is n-1.
function findExtra($arr1,
                   $arr2, $n)
{
for ($i = 0; $i < $n; $i++)
    if ($arr1[$i] != $arr2[$i])
        return $i;
 
return $n;
}
 
// Driver code
$arr1 = array (2, 4, 6, 8,
               10, 12, 13);
$arr2 = array(2, 4, 6,
              8, 10, 12);
$n = sizeof($arr2);
 
// Solve is passed
// both arrays
echo findExtra($arr1, $arr2, $n);
 
// This code is contributed by ajit
?>

Javascript




<script>
// JavaScript program to find an extra
// element present in arr1[]
 
// Returns index of extra element
// in arr1[]. n is size of arr2[].
// Size of arr1[] is n-1.
function findExtra(arr1, arr2, n)
{
for (let i = 0; i < n; i++)
    if (arr1[i] != arr2[i])
        return i;
 
return n;
}
 
// Driver code
 
    let arr1 = [2, 4, 6, 8,
                10, 12, 13];
    let arr2 = [2, 4, 6,
                8, 10, 12];
    let n = arr2.length;
 
    // Solve is passed both arrays
    document.write(findExtra(arr1, arr2, n));
 
// This code is contributed by Surbhi Tyagi.
</script>

Output : 

 6

Complexity Analysis: 

  • Time complexity: O(n). 
    As one traversal through the array is needed, so the time complexity is linear.
  • Space complexity: O(1). 
    Since no extra space is required, the time complexity is constant.

Method 2: This method is a better way to solve the above problem and uses the concept of binary search. 

Approach:To find the index of the missing element in less than linear time, binary search can be used, the idea is all the indices greater than or equal to the index of the missing element will have different elements in both the arrays and all the indices less than that index will have the similar elements in both arrays.

Algorithm: 

  1. Create three variables, low = 0, high = n-1, mid, ans = n
  2. Run a loop until low is less than or equal to high, i.e till our search range is less than zero.
  3. If the mid element, i.e (low + high)/2, of both arrays is similar then update the search to second half of the search range, i.e low = mid + 1
  4. Else update the search to the first half of the search range, i.e high = mid – 1, and update the answer to the current index, ans = mid
  5. Print the index.

Implementation: 



C++




// C++ program to find an extra
// element present in arr1[]
#include <iostream>
using namespace std;
 
// Returns index of extra element
// in arr1[]. n is size of arr2[].
// Size of arr1[] is n-1.
int findExtra(int arr1[],
              int arr2[], int n)
{
    // Initialize result
    int index = n;
 
    // left and right are end
    // points denoting the current range.
    int left = 0, right = n - 1;
    while (left <= right)
    {
        int mid = (left + right) / 2;
 
        // If middle element is same
        // of both arrays, it means
        // that extra element is after
        // mid so we update left to mid+1
        if (arr2[mid] == arr1[mid])
            left = mid + 1;
 
        // If middle element is different
        // of the arrays, it means that
        // the index we are searching for
        // is either mid, or before mid.
        // Hence we update right to mid-1.
        else
        {
            index = mid;
            right = mid - 1;
        }
    }
 
    // when right is greater than
    // left our search is complete.
    return index;
}
 
// Driver code
int main()
{
    int arr1[] = {2, 4, 6, 8, 10, 12, 13};
    int arr2[] = {2, 4, 6, 8, 10, 12};
    int n = sizeof(arr2) / sizeof(arr2[0]);
 
    // Solve is passed both arrays
    cout << findExtra(arr1, arr2, n);
    return 0;
}

Java




// Java program to find an extra
// element present in arr1[]
class GFG
{
    // Returns index of extra element
    // in arr1[]. n is size of arr2[].
    // Size of arr1[] is n-1.
    static int findExtra(int arr1[],
                         int arr2[], int n)
    {
        // Initialize result
        int index = n;
     
        // left and right are end
        // points denoting the current range.
        int left = 0, right = n - 1;
        while (left <= right)
        {
            int mid = (left+right) / 2;
     
            // If middle element is same
            // of both arrays, it means
            // that extra element is after
            // mid so we update left to mid+1
            if (arr2[mid] == arr1[mid])
                left = mid + 1;
     
            // If middle element is different
            // of the arrays, it means that
            // the index we are searching for
            // is either mid, or before mid.
            // Hence we update right to mid-1.
            else
            {
                index = mid;
                right = mid - 1;
            }
        }
     
        // when right is greater than
        // left, our search is complete.
        return index;
    }
     
    // Driver Code
    public static void main (String[] args)
    {
        int arr1[] = {2, 4, 6, 8, 10, 12,13};
        int arr2[] = {2, 4, 6, 8, 10, 12};
        int n = arr2.length;
     
        // Solve is passed both arrays
        System.out.println(findExtra(arr1, arr2, n));
    }
}
 
// This code is contributed by Harsh Agarwal

Python3




# Python3 program to find an extra
# element present in arr1[]
 
# Returns index of extra element
# in arr1[]. n is size of arr2[].
# Size of arr1[] is n-1.
def findExtra(arr1, arr2, n) :
 
    index = n # Initialize result
 
    # left and right are end points
    # denoting the current range.
    left = 0
    right = n - 1
    while (left <= right) :
        mid = (int)((left + right) / 2)
 
        # If middle element is same
        # of both arrays, it means
        # that extra element is after
        # mid so we update left to
        # mid + 1
        if (arr2[mid] == arr1[mid]) :
            left = mid + 1
 
        # If middle element is different
        # of the arrays, it means that
        # the index we are searching for
        # is either mid, or before mid.
        # Hence we update right to mid-1.
        else :
            index = mid
            right = mid - 1
         
    # when right is greater than left our
    # search is complete.
    return index
 
# Driver code
arr1 = [2, 4, 6, 8, 10, 12, 13]
arr2 = [2, 4, 6, 8, 10, 12]
n = len(arr2)
 
# Solve is passed both arrays
print(findExtra(arr1, arr2, n))
 
# This code is contributed by Nikita Tiwari.

C#




// C# program to find an extra
// element present in arr1[]
using System;
 
class GFG {
     
    // Returns index of extra
    // element in arr1[]. n is
    // size of arr2[].
    // Size of arr1[] is
    // n - 1.
    static int findExtra(int []arr1,
                         int []arr2,
                         int n)
    {
         
        // Initialize result
        int index = n;
     
        // left and right are
        // end points denoting
        // the current range.
        int left = 0, right = n - 1;
        while (left <= right)
        {
            int mid = (left+right) / 2;
     
            // If middle element is
            // same of both arrays,
            // it means that extra
            // element is after mid
            // so we update left
            // to mid + 1
            if (arr2[mid] == arr1[mid])
                left = mid + 1;
     
            // If middle element is
            // different of the arrays,
            // it means that the index
            // we are searching for is
            // either mid, or before mid.
            // Hence we update right to mid-1.
            else
            {
                index = mid;
                right = mid - 1;
            }
        }
     
        // when right is greater
        // than left our
        // search is complete.
        return index;
    }
     
    // Driver Code
    public static void Main ()
    {
        int []arr1 = {2, 4, 6, 8, 10, 12,13};
        int []arr2 = {2, 4, 6, 8, 10, 12};
        int n = arr2.Length;
     
        // Solve is passed
        // both arrays
        Console.Write(findExtra(arr1, arr2, n));
    }
}
 
// This code is contributed by nitin mittal.

PHP




<?php
// PHP program to find an extra
// element present in arr1[]
 
// Returns index of extra element
// in arr1[]. n is size of arr2[].
// Size of arr1[] is n-1.
function findExtra($arr1, $arr2, $n)
{
    // Initialize result
    $index = $n;
 
    // left and right are
    // end points denoting
    // the current range.
    $left = 0; $right = $n - 1;
    while ($left <= $right)
    {
        $mid = ($left+$right) / 2;
 
        // If middle element is same
        // of both arrays, it means
        // that extra element is after
        // mid so we update left to mid+1
        if ($arr2[$mid] == $arr1[$mid])
            $left = $mid + 1;
 
        // If middle element is different
        // of the arrays, it means that the
        // index we are searching for is either
        // mid, or before mid. Hence we update
        // right to mid-1.
        else
        {
            $index = $mid;
            $right = $mid - 1;
        }
    }
 
    // when right is greater than
    // left, our search is complete.
    return $index;
}
 
// Driver code
{
    $arr1 = array(2, 4, 6, 8,
                  10, 12, 13);
    $arr2 = array(2, 4, 6,
                  8, 10, 12);
    $n = sizeof($arr2) / sizeof($arr2[0]);
 
    // Solve is passed both arrays
    echo findExtra($arr1, $arr2, $n);
    return 0;
}
 
// This code is contributed by nitin mittal
?>

Javascript




<script>
 
 
// Javascript program to find an extra
// element present in arr1[]
 
// Returns index of extra element
// in arr1[]. n is size of arr2[].
// Size of arr1[] is n-1.
function findExtra( arr1, arr2, n)
{
    // Initialize result
    let index = n;
 
    // left and right are end
    // points denoting the current range.
    let left = 0, right = n - 1;
    while (left <= right)
    {
        let mid = Math.floor((left + right) / 2);
 
        // If middle element is same
        // of both arrays, it means
        // that extra element is after
        // mid so we update left to mid+1
        if (arr2[mid] == arr1[mid])
            left = mid + 1;
 
        // If middle element is different
        // of the arrays, it means that
        // the index we are searching for
        // is either mid, or before mid.
        // Hence we update right to mid-1.
        else
        {
            index = mid;
            right = mid - 1;
        }
    }
 
    // when right is greater than
    // left our search is complete.
    return index;
}
 
     
    // Driver program
     
    let arr1 = [2, 4, 6, 8, 10, 12, 13];
    let arr2 = [2, 4, 6, 8, 10, 12];
    let n = arr2.length;
 
    // Solve is passed both arrays
    document.write(findExtra(arr1, arr2, n));
     
     
</script>

Output : 

 6

Complexity Analysis: 

  • Time complexity : O(log n). 
    The time complexity of binary search is O(log n)
  • Space complexity : O(1). 
    As no extra space is required, so the time complexity is constant.

Method 3: This method solves the given problem using the predefined function. 
 

Approach: To find the element which is different, find the sum of each array and subtract the sums and find the absolute value. Search the larger array and check if the absolute is equal to an index and return that index. If an element is missing and all the other elements are the same, then the difference of sums will be equal to missing element.

Algorithm:

  1. Create a function to calculate the sum of two arrays.
  2. Find the absolute difference between the sum of two arrays (value).
  3. Traverse the larger array from start too end
  4. If the element at any index is equal to value, then print the index and break the loop.

Implementation: 

C++




// C++ code for above approach
#include<bits/stdc++.h>
using namespace std;
 
// function return sum of array elements
int sum(int arr[], int n)
{
    int summ = 0;
    for (int i = 0; i < n; i++)
    {
        summ += arr[i];
    }
    return summ;
}
 
// function return index of given element
int indexOf(int arr[], int element, int n)
{
    for (int i = 0; i < n; i++)
    {
        if (arr[i] == element)
        {
            return i;
        }
    }
    return -1;
}
 
// Function to find Index
int find_extra_element_index(int arrA[],
                             int arrB[],
                             int n, int m)
{
 
    // Calculating extra element
    int extra_element = sum(arrA, n) -
                        sum(arrB, m);
     
    // returns index of extra element
    return indexOf(arrA, extra_element, n);
}
 
// Driver Code
int main()
{
    int arrA[] = {2, 4, 6, 8, 10, 12, 13};
    int arrB[] = {2, 4, 6, 8, 10, 12};
    int n = sizeof(arrA) / sizeof(arrA[0]);
    int m = sizeof(arrB) / sizeof(arrB[0]);
    cout << find_extra_element_index(arrA, arrB, n, m);
}
 
// This code is contributed by mohit kumar

Java




// Java code for above approach
class GFG
{
 
    // Function to find Index
    static int find_extra_element_index(int[] arrA,
                                        int[] arrB)
    {
 
        // Calculating extra element
        int extra_element = sum(arrA) - sum(arrB);
         
        // returns index of extra element
        return indexOf(arrA, extra_element);
    }
     
    // function return sum of array elements
    static int sum(int[] arr)
    {
        int sum = 0;
        for (int i = 0; i < arr.length; i++)
        {
            sum += arr[i];
        }
        return sum;
    }
     
    // function return index of given element
    static int indexOf(int[] arr, int element)
    {
        for (int i = 0; i < arr.length; i++)
        {
            if (arr[i] == element)
            {
                return i;
            }
        }
        return -1;
    }
     
    // Driver Code
    public static void main(String[] args)
    {
        int[] arrA = {2, 4, 6, 8, 10, 12, 13};
        int[] arrB = {2, 4, 6, 8, 10, 12};
        System.out.println(find_extra_element_index(arrA, arrB));
    }
}
 
/* This code contributed by PrinciRaj1992 */

Python3




# Python3 code for above approach
 
# Function to find Index
def find_extra_element_index(arrA, arrB):
     
    # Calculating extra element
    extra_element = sum(arrA) - sum(arrB)
     
    # returns index of extra element
    return arrA.index(extra_element)
 
# Driver Code
arrA = [2, 4, 6, 8, 10, 12, 13]
arrB = [2, 4, 6, 8, 10, 12]
print(find_extra_element_index(arrA,arrB))
 
# This code is contributed by Dravid

C#




// C# code for above approach
using System;
 
class GFG
{
 
    // Function to find Index
    static int find_extra_element_index(int[] arrA,
                                        int[] arrB)
    {
 
        // Calculating extra element
        int extra_element = sum(arrA) - sum(arrB);
         
        // returns index of extra element
        return indexOf(arrA, extra_element);
    }
     
    // function return sum of array elements
    static int sum(int[] arr)
    {
        int sum = 0;
        for (int i = 0; i < arr.Length; i++)
        {
            sum += arr[i];
        }
        return sum;
    }
     
    // function return index of given element
    static int indexOf(int[] arr, int element)
    {
        for (int i = 0; i < arr.Length; i++)
        {
            if (arr[i] == element)
            {
                return i;
            }
        }
        return -1;
    }
     
    // Driver Code
    public static void Main(String[] args)
    {
        int[] arrA = {2, 4, 6, 8, 10, 12, 13};
        int[] arrB = {2, 4, 6, 8, 10, 12};
        Console.WriteLine(find_extra_element_index(arrA, arrB));
    }
}
 
// This code has been contributed by 29AjayKumar

Javascript




<script>
    // Javascript code for above approach
     
    // Function to find Index
    function find_extra_element_index(arrA, arrB)
    {
  
        // Calculating extra element
        let extra_element = sum(arrA) - sum(arrB);
          
        // returns index of extra element
        return indexOf(arrA, extra_element);
    }
      
    // function return sum of array elements
    function sum(arr)
    {
        let sum = 0;
        for (let i = 0; i < arr.length; i++)
        {
            sum += arr[i];
        }
        return sum;
    }
      
    // function return index of given element
    function indexOf(arr, element)
    {
        for (let i = 0; i < arr.length; i++)
        {
            if (arr[i] == element)
            {
                return i;
            }
        }
        return -1;
    }
     
    let arrA = [2, 4, 6, 8, 10, 12, 13];
    let arrB = [2, 4, 6, 8, 10, 12];
    document.write(find_extra_element_index(arrA, arrB));
     
</script>

Output : 

 6

Complexity Analysis: 

  • Time Complexity: O(n). 
    Since only three traversals through the array is needed, So the time complexity is linear.
  • Space Complexity: O(1). 
    As no extra space is required, so the time complexity is constant.

This article is contributed by Abhishek Khatri . If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
 




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