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# 18.4Sum

## 18. 4Sum

## 题目地址

<https://leetcode.com/problems/4sum/>

<https://www.lintcode.com/problem/4sum/description>

## 题目描述

```
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]
]
```

## 代码

### Approach #2 Two for-loops + Two points

```java
public class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
    List<List<Integer>> result = new ArrayList<List<Integer>>();
    if (nums == null || nums.length < 4) return result;
    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] == num[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 {
            ArrayList<Integer> tmp = new ArrayList<Integer>();
            tmp.add(nums[i]);
            tmp.add(nums[j]);
            tmp.add(nums[left]);
            tmp.add(nums[right]);
            result.add(tmp);
            left++;
            right--;
            while (left < right && nums[left] == nums[left - 1]) {
              left++;
            }
            while (left < right && nums[right] == nums[right + 1]) {
              right--;
            }
          }
        }
      }
    }

    return result;
  }
}
```


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