4Sum

leetcode 18.

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

Note: The solution set must not contain duplicate quadruplets.

For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0.

A solution set is:
[
  [-1,  0, 0, 1],
  [-2, -1, 1, 2],
  [-2,  0, 0, 2]
]

Solution. 3 Sum outer loop uses i, inner loop uses lo, hi. For 4 Sum, we need to use outer loop i, j. When i, j are fixed, we loop inner lo, hi.

In order to avoid the duplicate result, every time when we move i, j, lo, hi, we need to move them until to a different element.

public static List<List<Integer>> fourSum(int[] nums, int target) {
    List<List<Integer>> ans = new ArrayList<>();
    Arrays.sort(nums);
    int i = 0;
    while (i < nums.length - 3) {
        int j = i + 1;
        while (j < nums.length - 2) {
            int lo = j + 1, hi = nums.length - 1;
            while (lo < hi) {
                int sum = nums[i] + nums[j] + nums[lo] + nums[hi];
                if (sum == target) {
                    ans.add(Arrays.asList(nums[i], nums[j], nums[lo], nums[hi]));
                    while (++lo < hi && nums[lo] == nums[lo - 1]);  // one line technique to move lo until a different element.
                    while (lo < --hi && nums[hi] == nums[hi + 1]);
                }
                else if (sum > target) {
                    hi--;
                }
                else {
                    lo++;
                }
            }
            while (++j < nums.length - 2 && nums[j] == nums[j - 1]);
        }
        while (++i < nums.length - 3 && nums[i] == nums[i - 1]);
    }
    return ans;
}

Check my code on github.