【LeetCode】18. 4Sum

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.

Note:

The solution set must not contain duplicate quadruplets.

Example:

Given array nums = [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: Same idea as 3Sum or 2Sum

Code 1: 72ms

public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> res = new ArrayList<>();
        if (nums == null || nums.length < 4){
            return res;
        }
        Arrays.sort(nums);
        for (int i = 0; i < nums.length - 3; i++){
            if (i > 0 && nums[i] == nums[i - 1]) continue;
            for (int j = i + 1; j < nums.length - 2; j++){
                if (j > i + 1 && nums[j] == nums[j - 1]) continue;
                int l = j + 1, r = nums.length - 1;
                while (l < r){
                    if (nums[i] + nums[j] + nums[l] + nums[r] < target){
                        l++;
                    } else if (nums[i] + nums[j] + nums[l] + nums[r] > target){
                        r--;
                    } else {
                        res.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 res;
    }

Code 2: With enhancement, 26ms

public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> res = new ArrayList<>();
        if (nums == null || nums.length < 4){
            return res;
        }
        Arrays.sort(nums);
        for (int i = 0; i < nums.length - 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[nums.length - 1] + nums[nums.length - 2] + nums[nums.length - 3] < target) continue;
            for (int j = i + 1; j < nums.length - 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[nums.length - 1] + nums[nums.length - 2] < target) continue;
                int l = j + 1, r = nums.length - 1;
                while (l < r){
                    if (nums[i] + nums[j] + nums[l] + nums[r] < target){
                        l++;
                    } else if (nums[i] + nums[j] + nums[l] + nums[r] > target){
                        r--;
                    } else {
                        res.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 res;
    }