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# 15. 3Sum

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]
]
```

#### 解题要点：

先从头开始遍历，然后同时用双指针处理找后面是否有符合条件的数字，加入到最终返回值里。（剪枝：重复的不作处理，直接跳过）

```python
class Solution(object):
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        res = []
        nums.sort()
        for i in range(len(nums)-2):
            if i > 0 and nums[i] == nums[i-1]:
                continue
            l = i + 1
            h = len(nums) - 1
            temp = 0 - nums[i]
            while l < h:
                if nums[l] + nums[h] > temp:
                    h -= 1
                elif nums[l] + nums[h] < temp:
                    l += 1
                else:
                    res.append([nums[i], nums[l], nums[h]])
                    while l < h and nums[l] == nums[l+1]: l += 1
                    while l < h and nums[h] == nums[h-1]: h -= 1
                    l += 1
                    h -= 1
        
        return res
```