Find Pairs in Array with Given Sum in C++

March 29, 2023

Pairs in Array with given Sum

On this page, we will look into a coding question where we will learn how to Find Pairs in Array with given sum in C++ Programing Language. There might be different approaches to solve this question, one you will find here. If your approach is a bit different post it in the comment section.

Find Pairs in Array with given Sum in C++

To find pairs in an array with a given sum in C, we can use two methods

Brute Force (Time complexity: O(n^2) )
Using Sorting (Time complexity: O(n log n) )

Both methods have their own pros and cons. The brute force method can be slow for large arrays but it is very simple and easy to implement . The sorting method is faster for large arrays, but requires extra space for sorting and modifying the original array.

Method 1 : Brute Force Method

In this approach, we check every pair of elements in the array to see if their sum is equal to the given target sum.
This method has a time complexity of O(n^2) as we have to compare each element with every other element

Introduction to 2-D Arrays

Reverse an array or string

Majority Element in Array

Find pairs in array with given sum

Sort the array in Waveform

C++ code for brute force

Run

#include <iostream>
using namespace std;

void findPairs (int arr[], int n, int targetSum)
{
  for (int i = 0; i < n - 1; i++)
    {
      for (int j = i + 1; j < n; j++)
	{
	  if (arr[i] + arr[j] == targetSum)
	    {
	      cout << "Pair found at index " << i << " and " << j << endl;
	    }
	}
    }
}

int main ()
{
  int arr[] = { 5, 8, 1, 4, 6, 3, 2, 7 };
  int n = sizeof (arr) / sizeof (arr[0]);
  int targetSum = 10;
  findPairs (arr, n, targetSum);
  return 0;
}

Output

Pair found at index 1 and 6
Pair found at index 3 and 4
Pair found at index 5 and 7

Method 2 : Sorting Method

The array is first sorted in ascending order in the sorting technique.
Then, we employ two pointers, one of which points to the first element and the other to the last element.
These two pointers values are added, and we then check to see if the result equals the specified sum.
The left pointer is increased if the value is less than the total, and the right pointer is decreased if the value is higher.
This method has a time complexity of O(n log n) due to the sorting algorithm used.

C++ code for sorting method

Run

#include <iostream>
#include <algorithm>
using namespace std;

void findPairs (int arr[], int n, int targetSum)
{
  sort (arr, arr + n);

  int left = 0;
  int right = n - 1;

  while (left < right)
    {
      int currSum = arr[left] + arr[right];

      if (currSum == targetSum)
	{
	  cout << "Pair found: " << arr[left] << " and " << arr[right] <<
	    endl;
	  left++;
	  right--;
	}
      else if (currSum < targetSum)
	{
	  left++;
	}
      else
	{
	  right--;
	}
    }
}


int main ()
{
  int arr[] = { 5, 8, 1, 4, 6, 3, 2, 7 };
  int n = sizeof (arr) / sizeof (arr[0]);
  int targetSum = 10;
  findPairs (arr, n, targetSum);
  return 0;
}

Output

Pair found: 2 and 8
Pair found: 3 and 7
Pair found: 4 and 6

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