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]
]

和3sum几乎一样

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
          List<List<Integer>> results = new ArrayList<>();
     if(nums.length < 4) return results;
    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 left = j + 1, right = nums.length - 1;
            while(left < right){
            if(target == nums[i] + nums[j]+nums[left]+nums[right]){
              List<Integer> result = Arrays.asList(nums[i],nums[j], nums[left], nums[right]);
                results.add(result);
                while(left < right && nums[left]==nums[left+1]) left++;
                while(left < right && nums[right]==nums[right-1]) right--;
                left++;
                right--;
            }
            else if(target < nums[i] + nums[j]+nums[left]+nums[right]){
                right--;
            }
            else{
                left++;
            }
            }
        }
    }
    return results;
    }
}