18. 4Sum.js

var fourSum = function (nums, target) {
    nums.sort((a, b) => a - b);
    const res = [];
    const n = nums.length;

    for (let i = 0; i < n - 3; i++) {
        if (i > 0 && nums[i] === nums[i - 1]) continue; // skip duplicates for i

        for (let j = i + 1; j < n - 2; j++) {
            if (j > i + 1 && nums[j] === nums[j - 1]) continue; // skip duplicates for j

            let left = j + 1, right = n - 1;

            while (left < right) {
                const sum = nums[i] + nums[j] + nums[left] + nums[right];

                if (sum === target) {
                    res.push([nums[i], nums[j], nums[left], nums[right]]);
                    left++;
                    right--;

                    // skip duplicates for left and right
                    while (left < right && nums[left] === nums[left - 1]) left++;
                    while (left < right && nums[right] === nums[right + 1]) right--;

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

    return res;
};