Count pairs that add up to a given sum “k”.

function findPairs(arr, k) {
    let numOfPairs = 0;
    
    // 'numerOfPairs' will return double the actual number of pairs because of nested iteration
    for (let i = 0; i < arr.length; i++) {
        for (let j = 0; j < arr.length; j++) {
            if (arr[i] + arr[j] === k) {
                numOfPairs++;
            }
        }
    }
    
    // dividing 'numOfPairs' by 2 to avoid duplicate results
    let actualNumOfPairs = numOfPairs/2;
    
    return actualNumOfPairs;
}
const arr = [1, 5, 5, 7, -1];
const k = 6;
const result = findPairs(arr, k);
console.log(result);

function findAddingPairs(nums, k) {
    let count = 0;
    let mp = new Map();
    
    for (let i = 0; i < nums.length; i++) {
        let b = k - nums[i];
        // increasing 'count' by number of previous occurences of 'b'
        if (mp.has(b)) {
            count += mp.get(b);
        }
        
        if (mp.has(nums[i])) {
            // if 'nums[i]' exists already => incrementing the value/frequency by 1
            mp.set(nums[i], mp.get(nums[i])+1);
        } else {
            // if 'nums[i]' doesn't exist => set as new value
            mp.set(nums[i], 1);
        }
    }
    
    return count;
};

const arr = [1, 2, 3, 4, 0, -1];
const k = 3;
const result = findAddingPairs(arr, k);
console.log(result);