第二天 3 Sum

这道题是昨天就想好要来刷的，继续用Python，感觉通过刷题，来对这门语言有点感觉，也是个不错的方法吧

https://leetcode-cn.com/problems/3sum/description/

题目在昨天的两数之和的基础上其实难度还是增加了不少。

最初拿到这道题的思路，其实就是希望将问题“退化”到两数之和。依次遍历给定的列表，取出的值先取反，如果剩下的列表中有两数之和等于这个取反之后的值，就构成了一组解。至于去重的话，就干脆用了set，自己就没有再深入做了

为了遍历和后续处理数据方便，上来就需要把列表排个序【反正这次不需要返回在原来数组的索引值了】

接着当取出一个值后，“退化”为两数之和的时候，因为是有顺序的，那么就可以相当用两个指针，分别指向一头一尾，采取两头夹逼的方法。这里特别需要注意的时候，当找到一组解之后，还需要再继续寻找。

那么根据这个思路，我就写出了这样一段代码：

class Solution:
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        ret = set()
        nums.sort()
        for k in range(len(nums)):
            i = k+1
            j = len(nums)-1
            target = -nums[k]
            while i<j:
                two_sum = nums[i] + nums[j]
                if two_sum == target:
                    ret.add((nums[k],nums[i],nums[j]))
                    i+=1
                    j-=1
                elif two_sum > target:
                    j-=1
                elif two_sum < target:
                    i+=1
        return list(ret)

结果很遗憾，TLE了
没过的case：

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开始本来认为是两侧夹逼的思路不如昨天那种字典的思路，但后来再仔细看了一下这个跑挂了的测试用例，才觉得问题应该是出现在是否需要退化到“二数之和”的部分，如果列表中两个值相等，那么前一个是否有结果，与后面一个应该是一样的，那么也就不需要循环了。
另外还有个小的潜在的优化点，就是循环的时候，只需要到倒数第三个值就可以了。
好，那么简单修改后的代码如下：

class Solution:
    def threeSum(self, nums):
        """
        :type nums: List[int]
        :rtype: List[List[int]]
        """
        ret = set()
        nums.sort()
        for k in range(len(nums)-2):
            if (k>0 and nums[k] == nums[k-1]):
                continue
            i = k+1
            j = len(nums)-1
            target = -nums[k]
            while i<j:
                two_sum = nums[i] + nums[j]
                if two_sum == target:
                    ret.add((nums[k],nums[i],nums[j]))
                    i+=1
                    j-=1
                elif two_sum > target:
                    j-=1
                elif two_sum < target:
                    i+=1
        return list(ret)

这样修改后，可以AC飘过，但也还要1秒多，看到leetcode上最好的算法只要用400ms，当然如果换成Java，“平铺直叙”的写法也只要100ms以内，这。。。。。。

好吧，其实我就是为了学习下Python而已

今天对于set的用法稍微实践了一下，另外遍历了的时候也可以使用range这种方法。