Sum of the 4

Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

The solution set must not contain duplicate quadruplets.

Example:

Input: nums = [1, 0, -1, 0, -2, 2], target = 0
Output:
[
  [-1,  0, 0, 1],
  [-2, -1, 1, 2],
  [-2,  0, 0, 2]
]

Solution

Complexity:

• Time complexity: O(n^3)
• Space complexity: O(1)

function fourSum(nums, target) {
    if (nums.length < 4) {
        return [];
    }

    const len = nums.length;
    const res = [];
    let l = 0;
    let r = 0;
    let sum = 0;

    nums.sort((a, b) => a - b);

    for (let i = 0; i < len - 3; i++) {
        if (i > 0 && nums[i] === nums[i - 1]) continue;
        if (nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3] > target) break;
        if (nums[i] + nums[len - 1] + nums[len - 2] + nums[len - 3] < target) continue;

        for (let j = i + 1; j < len - 2; j++) {
            if (j > i + 1 && nums[j] === nums[j - 1]) continue;
            if (nums[i] + nums[j] + nums[j + 1] + nums[j + 2] > target) break;
            if (nums[i] + nums[j] + nums[len - 1] + nums[len - 2] < target) continue;

            l = j + 1;
            r = len - 1;

            while (l < r) {
                sum = nums[i] + nums[j] + nums[l] + nums[r];

                if (sum < target) {
                    l++;
                } else if (sum > target) {
                    r--;
                } else {
                    res.push([nums[i], nums[j], nums[l], nums[r]]);
                    while (l < r && nums[l] === nums[l + 1]) l++;
                    while (l < r && nums[r] === nums[r - 1]) r--;
                    l++;
                    r--;
                }
            }
        }
    }

    return res;
}