Minimum swaps of same-indexed elements required to make sum of two given arrays even

Last Updated : 18 Nov, 2021

Given two arrays arr1[] and arr2[] of size N, the task is to count the minimum number of swaps of same-indexed elements from both the arrays arr1[] and arr2[] required to make the sum of all elements of both the arrays even. If it is not possible, then print “-1”.

Examples:

Input: arr1[] = {1, 4, 2, 3}, arr2[] = {2, 3, 4, 1}
Output: 0
Explanation: Sum of all elements of arr1[] and arr2[] is 10 and 10 respectively, which is already even. Therefore, the count of operations required is 0.

Input: arr1[] = {11, 14, 20, 2}, arr2[] = {5, 9, 6, 3}
Output: 1
Explanation: Sum of all elements of arr1[] and arr2[] is 37 and 23 respectively. Swapping arr1[1]( = 14) and arr2[1]( = 9) makes the sum of arr1[] and arr2[], 32 and 28 respectively. Therefore, the count of operations required is 1.

Approach: The idea is based on the following observations assuming that the sum of the array arr1[] is sumArr1 and that of arr2[] is sumArr2.

If sumArr1 is even and sumArr2 is even: No swaps required.
If sumArr1 is odd and sumArr2 is odd: Find a pair of same-indexed elements whose sum is odd and swap them. Such a pair contains one even number and an odd number. Swapping them increases the sum of one array by 1 and decreases that of the other array by 1. Therefore, sum of both the arrays is even.
If sumArr1 is even and sumArr2 is odd: Not possible to make sum of both the arrays even.
If sumArr1 is odd and sumArr2 is even: Not possible to make sum of both the arrays even.

Follow the steps below to solve the problem:

Initialize sumArr1 = 0 and sumArr2 = 0 to store the sum of arr1[] and arr2[] respectively.
If sumArr1 and sumArr2 are both found to be even, then print 0.
If sumArr1 and sumArr2 are both found to be odd, then iterate a loop over the range [0, N – 1] and check if there exists any corresponding pair whose sum is odd or not. If any such pair is found, then print 1.
Otherwise, for all other cases, print -1.

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to count the minimum swaps
// of same-indexed elements from arrays
// arr1[] and arr2[] required to make
// the sum of both the arrays even
void minimumSwaps(int arr1[], int arr2[],
                  int n)
{
    // Store the sum of elements of
    // the array arr1 and arr2 respectively
    int sumArr1 = 0, sumArr2 = 0;
 
    // Store the array sum of both the arrays
    for (int i = 0; i < n; ++i) {
        sumArr1 += arr1[i];
        sumArr2 += arr2[i];
    }
 
    // If both sumArr1 and sumArr2
    // are even, print 0 and return
    if (sumArr1 % 2 == 0
        && sumArr2 % 2 == 0) {
        cout << 0;
        return;
    }
 
    // If both sumArr1 and sumArr2
    // are odd and check for a pair
    // with sum odd sum
    if (sumArr1 % 2 != 0
        && sumArr2 % 2 != 0) {
 
        // Stores if a pair with
        // odd sum exists or not
        int flag = -1;
 
        // Traverse the array
        for (int i = 0; i < n; ++i) {
 
            // If a pair exists with odd
            // sum, set flag = 1
            if ((arr1[i] + arr2[i]) % 2 == 1){
                flag = 1;
                break;
            }
        }
 
        // Print the answer and return
        cout << flag;
 
        return;
    }
 
    // For all other cases, print -1
    cout << -1;
}
 
// Driver Code
int main()
{
    int arr1[] = { 11, 14, 20, 2 };
    int arr2[] = { 5, 9, 6, 3 };
    int N = sizeof(arr1) / sizeof(arr1[0]);
 
    // Function Call
    minimumSwaps(arr1, arr2, N);
 
    return 0;
}

Java

// Java program for the above approach
import java.io.*;
 
class GFG{
 
// Function to count the minimum swaps
// of same-indexed elements from arrays
// arr1[] and arr2[] required to make
// the sum of both the arrays even
static void minimumSwaps(int arr1[], int arr2[],
                         int n)
{
     
    // Store the sum of elements of
    // the array arr1 and arr2 respectively
    int sumArr1 = 0, sumArr2 = 0;
 
    // Store the array sum of both the arrays
    for(int i = 0; i < n; ++i)
    {
        sumArr1 += arr1[i];
        sumArr2 += arr2[i];
    }
 
    // If both sumArr1 and sumArr2
    // are even, print 0 and return
    if (sumArr1 % 2 == 0 && sumArr2 % 2 == 0)
    {
        System.out.print(0);
        return;
    }
 
    // If both sumArr1 and sumArr2
    // are odd and check for a pair
    // with sum odd sum
    if (sumArr1 % 2 != 0 && sumArr2 % 2 != 0) 
    {
         
        // Stores if a pair with
        // odd sum exists or not
        int flag = -1;
 
        // Traverse the array
        for(int i = 0; i < n; ++i) 
        {
             
            // If a pair exists with odd
            // sum, set flag = 1
            if ((arr1[i] + arr2[i]) % 2 == 1)
            {
                flag = 1;
                break;
            }
        }
 
        // Print the answer and return
        System.out.print(flag);
        return;
    }
 
    // For all other cases, print -1
    System.out.print(-1);
}
 
// Driver code
public static void main(String[] args)
{
    int arr1[] = { 11, 14, 20, 2 };
    int arr2[] = { 5, 9, 6, 3 };
    int N = arr1.length;
     
    // Function Call
    minimumSwaps(arr1, arr2, N);
}
}
 
// This code is contributed by jithin

Python3

# Python program for the above approach
 
# Function to count the minimum swaps
# of same-indexed elements from arrays
# arr1 and arr2 required to make
# the sum of both the arrays even
def minimumSwaps(arr1, arr2, n):
   
    # Store the sum of elements of
    # the array arr1 and arr2 respectively
    sumArr1 = 0; sumArr2 = 0;
 
    # Store the array sum of both the arrays
    for i in range(n):
        sumArr1 += arr1[i];
        sumArr2 += arr2[i];
 
    # If both sumArr1 and sumArr2
    # are even, pr0 and return
    if (sumArr1 % 2 == 0 and sumArr2 % 2 == 0):
        print(0);
        return;
 
    # If both sumArr1 and sumArr2
    # are odd and check for a pair
    # with sum odd sum
    if (sumArr1 % 2 != 0 and sumArr2 % 2 != 0):
 
        # Stores if a pair with
        # odd sum exists or not
        flag = -1;
 
        # Traverse the array
        for i in range(n):
 
            # If a pair exists with odd
            # sum, set flag = 1
            if ((arr1[i] + arr2[i]) % 2 == 1):
                flag = 1;
                break;
 
        # Print the answer and return
        print(flag);
        return;
 
    # For all other cases, pr-1
    print(-1);
 
# Driver code
if __name__ == '__main__':
    arr1 = [11, 14, 20, 2];
    arr2 = [5, 9, 6, 3];
    N = len(arr1);
 
    # Function Call
    minimumSwaps(arr1, arr2, N);
 
    # This code is contributed by 29AjayKumar 

C#

// C# program to implement
// the above approach  
using System;
class GFG{
  
// Function to count the minimum swaps
// of same-indexed elements from arrays
// arr1[] and arr2[] required to make
// the sum of both the arrays even
static void minimumSwaps(int[] arr1, int[] arr2,
                         int n)
{
      
    // Store the sum of elements of
    // the array arr1 and arr2 respectively
    int sumArr1 = 0, sumArr2 = 0;
  
    // Store the array sum of both the arrays
    for(int i = 0; i < n; ++i)
    {
        sumArr1 += arr1[i];
        sumArr2 += arr2[i];
    }
  
    // If both sumArr1 and sumArr2
    // are even, print 0 and return
    if (sumArr1 % 2 == 0 && sumArr2 % 2 == 0)
    {
        Console.Write(0);
        return;
    }
  
    // If both sumArr1 and sumArr2
    // are odd and check for a pair
    // with sum odd sum
    if (sumArr1 % 2 != 0 && sumArr2 % 2 != 0) 
    {
          
        // Stores if a pair with
        // odd sum exists or not
        int flag = -1;
  
        // Traverse the array
        for(int i = 0; i < n; ++i) 
        {
              
            // If a pair exists with odd
            // sum, set flag = 1
            if ((arr1[i] + arr2[i]) % 2 == 1)
            {
                flag = 1;
                break;
            }
        }
  
        // Print the answer and return
        Console.Write(flag);
        return;
    }
  
    // For all other cases, print -1
    Console.Write(-1);
}
  
// Driver code
public static void Main()
{
    int[] arr1 = { 11, 14, 20, 2 };
    int[] arr2 = { 5, 9, 6, 3 };
    int N = arr1.Length;
      
    // Function Call
    minimumSwaps(arr1, arr2, N);
}
}
 
// This code is contributed by susmitakundugoaldanga

Javascript

<script>
 
// Javascript program for the above approach
 
// Function to count the minimum swaps
// of same-indexed elements from arrays
// arr1[] and arr2[] required to make
// the sum of both the arrays even
function minimumSwaps(arr1, arr2, n)
{
     
    // Store the sum of elements of
    // the array arr1 and arr2 respectively
    let sumArr1 = 0, sumArr2 = 0;
 
    // Store the array sum of both the arrays
    for(let i = 0; i < n; ++i)
    {
        sumArr1 += arr1[i];
        sumArr2 += arr2[i];
    }
 
    // If both sumArr1 and sumArr2
    // are even, print 0 and return
    if (sumArr1 % 2 == 0 && sumArr2 % 2 == 0)
    {
        document.write(0);
        return;
    }
 
    // If both sumArr1 and sumArr2
    // are odd and check for a pair
    // with sum odd sum
    if (sumArr1 % 2 != 0 && sumArr2 % 2 != 0) 
    {
         
        // Stores if a pair with
        // odd sum exists or not
        let flag = -1;
 
        // Traverse the array
        for(let i = 0; i < n; ++i) 
        {
             
            // If a pair exists with odd
            // sum, set flag = 1
            if ((arr1[i] + arr2[i]) % 2 == 1)
            {
                flag = 1;
                break;
            }
        }
 
        // Print the answer and return
        document.write(flag);
        return;
    }
 
    // For all other cases, print -1
    document.write(-1);
}
 
// Driver Code
let arr1 = [ 11, 14, 20, 2 ];
let arr2 = [ 5, 9, 6, 3 ];
let N = arr1.length;
 
// Function Call
minimumSwaps(arr1, arr2, N);
 
// This code is contributed by splevel62
 
</script>

Output:

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

Minimum number of swaps required to make parity of array elements same as their indices

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Minimum swaps of same-indexed elements required to make sum of two given arrays even

C++

Java

Python3

C#

Javascript

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