LeetCode 16. 3Sum Closest (javascript)

Given an array nums of n integers and an integer target, find three integers in nums such that the sum is closest to target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

Example 1:

Input: nums = [-1,2,1,-4], target = 1
Output: 2
Explanation: The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

Constraints:

• 3 <= nums.length <= 10^3
• -10^3 <= nums[i] <= 10^3
• -10^4 <= target <= 10^4

Idea:

Sorting the array + two pointers traversal

Solution:

/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number}
 */
var threeSumClosest = function(nums, target) {
    if (nums === null || nums.length <= 2) return null;
    if (nums.length === 3) return nums[0] + nums[1] + nums[2];
    // sort array first
    nums.sort((a, b) => a - b);
    let n = nums.length;
    let res = null;
    let closest = Infinity;
    // two pointers traverse
    for (let i = 0; i < n; i++) {
        let j = i + 1;
        let k = n - 1;
        while (j < k) {
            let sum = nums[i] + nums[j] + nums[k];
            let diff = sum - target;
            if (diff === 0) {
                return sum;
            } else if (diff > 0) {
                // too large, make it smaller by moving k to the left
                k--;
            } else {
                // get the postive diff
                diff = target - sum;
                // too small, make it larger by moving j to the right;
                j++;
            }
            // keep tracking the current closest and update the current sum
            if (diff < closest) {
                closest = diff;
                res = sum;
            }
        }
    }
    return res;
}