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# 15.3Sum

## 题目描述

Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.
Note:
The solution set must not contain duplicate triplets.
Example:
Given array nums = [-1, 0, 1, 2, -1, -4],
A solution set is:
[
[-1, 0, 1],
[-1, -1, 2]
]

## 代码

### Approach 1: 排序 + 2 Sum

class Solution {
public List<List<Integer>> threeSum(int[] nums) {
List<List<Integer>> result = new ArrayList<List<Integer>>();
Arrays.sort(nums);
for (int i = 0; i < nums.length - 2; i++) {
if (i > 0 && nums[i] == nums[i - 1]) {
continue;
}
int left = i + 1;
int right = nums.length - 1;
while (left < right) {
int tmp = nums[left] + nums[right]; // two sum
if (tmp + nums[i] == 0) {
result.add(Arrays.asList(nums[i], nums[left], nums[right]));
left++;
// 去重
while (left < right && nums[left] == nums[left - 1]) {
left++;
}
} else if (tmp + nums[i] < 0) {
left++;
} else {
right--;
}
}
}
return result;
}
}