count_pairs_whose_sum_is_less_than_target.ts

/*
2824. Count Pairs Whose Sum is Less than Target

Given a 0-indexed integer array nums of length n and an integer target, 
 return the number of pairs (i, j) where 0 <= i < j < n and nums[i] + nums[j] < target.
 

Example 1:

Input: nums = [-1,1,2,3,1], target = 2
Output: 3
Explanation: There are 3 pairs of indices that satisfy the conditions in the statement:
- (0, 1) since 0 < 1 and nums[0] + nums[1] = 0 < target
- (0, 2) since 0 < 2 and nums[0] + nums[2] = 1 < target 
- (0, 4) since 0 < 4 and nums[0] + nums[4] = 0 < target
Note that (0, 3) is not counted since nums[0] + nums[3] is not strictly less than the target.

*/

function countPairs(nums: number[], target: number): number {
    nums.sort((a,b) => a-b);
    let count = 0;
    let left = 0;
    let right = nums.length-1;
    
    while(left < right) {
        const sum = nums[left] + nums[right];

        if (sum < target) {
            count += right - left;
            left++;
        } else {
            right--;
        }
    }

    return count;
};

const result = countPairs([-1,1,2,3,1], 2);
console.log(result);