[数据结构与算法]15. 三数之和

思想很简单：排序+三指针

就是要注意怎么减少循环次数，不然疯狂超时。主要考虑到第一个指针指向的值若是已经大于0，就可以结束了；第二个指针和第三个指针当前值若与下一个指向的值相等，也可以直接移动了，不用计算和比较了。

class Solution:
    def threeSum(self, nums: List[int]) -> List[List[int]]:
        n = len(nums)
        if not nums or n < 3:
            return []
        nums.sort()
        result = []
        for first in range(n):
            if nums[first] > 0:
                return result
            if first > 0 and nums[first] == nums[first-1]:
                continue
            second, third = first+1, n-1
            while second < third:
                temp_sum = nums[first] + nums[second] + nums[third]
                if temp_sum == 0:
                    result.append([nums[first], nums[second], nums[third]])
                    while second < third and nums[second] == nums[second+1]:
                        second += 1
                    while second < third and nums[third] == nums[third-1]:
                        third -= 1
                    second += 1
                    third -= 1
                elif temp_sum > 0:
                    third -= 1
                else:
                    second += 1
        return result

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