Javascript Program to Count pairs with given sum

Given an array of integers, and a number 'sum', find the number of pairs of integers in the array whose sum is equal to 'sum'.
Note: Duplicate pairs are also allowed.

Examples:  

Input  :  arr[] = {1, 5, 7, -1}, 
          sum = 6
Output :  2
Pairs with sum 6 are (1, 5) and (7, -1)

Input  :  arr[] = {1, 5, 7, -1, 5}, 
          sum = 6
Output :  3
Pairs with sum 6 are (1, 5), (7, -1) &
                     (1, 5)         

Input  :  arr[] = {1, 1, 1, 1}, 
          sum = 2
Output :  6
There are 3! pairs with sum 2.

Input  :  arr[] = {10, 12, 10, 15, -1, 7, 6, 
                   5, 4, 2, 1, 1, 1}, 
          sum = 11
Output :  9

Expected time complexity O(n)

Naive Solution :

A simple solution is to traverse each element and check if there's another number in the array which can be added to it to give sum. 

// Javascript implementation of simple method to find count of
// pairs with given sum.

// Returns number of pairs in arr[0..n-1] with sum equal
// to 'sum'
function getPairsCount(arr, n, sum) {
    let count = 0; // Initialize result

    // Consider all possible pairs and check their sums
    for (let i = 0; i < n; i++)
        for (let j = i + 1; j < n; j++)
            if (arr[i] + arr[j] == sum)
                count++;

    return count;
}

// Driver function to test the above function
let arr = [1, 5, 7, -1, 5];
let n = arr.length;
let sum = 6;
console.log("Count of pairs is " + getPairsCount(arr, n, sum));

// This code is contributed by Mayank Tyagi

Count of pairs is 3

Complexity Analysis:

• Time Complexity: O(n²) 
• Auxiliary Space: O(1)

Please refer complete article on Count pairs with given sum for more details!