Leetcode 18. 4Sum

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]
]

public class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> res = new ArrayList<>();
        Set<List<Integer>> set = new HashSet<>();
        if (nums.length < 4) return res;
        int len = nums.length;
        Arrays.sort(nums);
        
        for (int i = 0; i < len - 3; i++) {
            for (int j = i + 1; j < len - 2; j++) {
                int start = j + 1;
                int end = len - 1;
                while (start < end) {
                    int sum = nums[i] + nums[j] + nums[start] + nums[end];
                    if (sum == target) {
                        set.add(Arrays.asList(nums[i], nums[j], nums[start], nums[end]));
                        while (start < end && nums[start] == nums[start + 1]) start++;
                        while (start < end && nums[end] == nums[end - 1]) end--;
                        start++;
                        end--;
                    }
                    else if (sum < target) start++;
                    else end--;
                }
            }
        }
        return new ArrayList<>(set);
    }
}

第二种方法稍微快一点

    public List<List<Integer>> fourSum(int[] nums, int target) {
        Arrays.sort(nums);
        List<List<Integer>> res = new ArrayList<>();
        
        for(int i =0 ;i < nums.length - 3; i ++){
            for(int j = i + 1; j < nums.length - 2; j ++){
                int left = j + 1, right = nums.length - 1, sum = target - nums[i] - nums[j];
                
                while(left < right){
                    if(nums[left] + nums[right] == sum){
                        List<Integer> item = Arrays.asList(nums[i], nums[j], nums[left++], nums[right--]);
                        if(!res.contains(item)){
                            res.add(item);
                        }
                    }else if(nums[left] + nums[right] < sum){
                        left ++;
                    }else{
                        right --;
                    }
                }
            }
        }
        
        return res;