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]
]

題目

思路

1. Sort the array (coz we need to check the duplicates) 

2. Lock two pointer on left most side and do two sum with the other two in the remaing

程式碼

 1 class Solution {
 2        public List<List<Integer>> fourSum(int[] nums, int target) {
 3         List<List<Integer>> result = new ArrayList<>();
 4         if (nums.length < 4) return result;
 5         Arrays.sort(nums);
 6         for (int i = 0; i < nums.length - 3; ++i) {
 
 7             if (i > 0 && nums[i] == nums[i-1]) continue;
 8             for (int j = i + 1; j < nums.length - 2; ++j) {
 9                 if (j > i+1 && nums[j] == nums[j-1]) continue;
10                 int k = j + 1;
11                 int l = nums.length - 1;
12                 while (k < l) {
13                     final int sum = nums[i] + nums[j] + nums[k] + nums[l];
14                     if (sum < target) {
15                         ++k;
16                         while(nums[k] == nums[k-1] && k < l) ++k;
17                     } else if (sum > target) {
18                         --l;
19                         while(nums[l] == nums[l+1] && k < l) --l;
20                     } else {
21                         result.add(Arrays.asList(nums[i], nums[j], nums[k], nums[l]));
22                         ++k;
23                         --l;
24                         while(nums[k] == nums[k-1] && k < l) ++k;
25                         while(nums[l] == nums[l+1] && k < l) --l;
26                     }
27                 }
28             }
29         }
30         return result;
31     }
32 }