4 Sum

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.

• Time: O(n^3) Space: O(1)

public List<List<Integer>> fourSum(int[] nums, int target) {
    Arrays.sort(nums);
    List<List<Integer>> ret = new ArrayList<>();
    for (int i = 0; i < nums.length; i++) {
        if (i > 0 && nums[i] == nums[i - 1]) continue;
        for (int j = i + 1; j < nums.length; j++) {
            if (j > i + 1 && nums[j] == nums[j - 1]) continue;
            int l = j + 1, r = nums.length - 1;
            while (l < r) {
                int sum = nums[i] + nums[j] + nums[l] + nums[r];
                if (sum > target) {
                    r--;
                } else if (sum < target) {
                    l++;
                } else {
                    ret.add(Arrays.asList(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 ret;
}