LeetCode 18. 4Sum 四数之和

题目

https://leetcode.com/problems/4sum/description/

Given an array `nums` of n integers and an integer `target`, are there elements abc, 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]
]
```

Java参考代码

```class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
List<List<Integer>> result = new ArrayList<>();
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;
int right = nums.length - 1;

while (left < right) {
int sum = nums[i] + nums[j] + nums[left] + nums[right];

if (sum < target) {
left++;
} else if (sum > target) {
right--;
} else {

left++;
right--;

while (left < right && nums[left] == nums[left - 1]) {
left++;
}

while (left < right && nums[right] == nums[right + 1]) {
right--;
}
}
}
}
}

return result;

// Time Complexity: O(n^3)
// Space Complexity: O(n^3)
}
}```