15 Three Sum

思路: 最外层套一个循环，里面是Two Sum， 排序之后使用two pointer 来解，排序之后还有一个好处，就是在枚举和移动指针的时候，相等的值可以跳过，这样可以直接去重。

class Solution:
    def threeSum(self, nums):
        nums.sort()
        results = []
        length = len(nums)
        for i in range(0, length - 2):
            if i and nums[i] == nums[i - 1]:
                continue
            self.find_two_sum(nums, i + 1, length - 1, -nums[i], results)
        return results

    def find_two_sum(self, nums, left, right, target, results):
        while left < right:
            if nums[left] + nums[right] == target:
                results.append([-target, nums[left], nums[right]])
                right -= 1
                left += 1
                while left < right and nums[left] == nums[left - 1]:
                    left += 1
                while left < right and nums[right] == nums[right + 1]:
                    right -= 1
            elif nums[left] + nums[right] > target:
                right -= 1
            else:
                left += 1