LeetCode 四数之和算法题优化（ Python ）

[ChenJHua]

https://leetcode-cn.com/problems/4sum/description/

我的代码，提交解答的时候通不过，显示超出时间限制，请大佬帮我优化一下：

class Solution:
    def fourSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[List[int]]
        """
        res, dicti = set(), {}
        numLen = len(nums)
        nums.sort()
        for i in range(numLen):
            for j in range(i+1, numLen):
                key = nums[i] + nums[j]
                if key not in dicti.keys():
                    dicti[key] = [(i,j)]
                else:
                    dicti[key].append((i,j))
        for i in range(numLen):
            for j in range(i+1, numLen-2):
                exp = target - nums[i] -nums[j]
                if exp in dicti.keys():
                    for tmpIndex in dicti[exp]:
                        if tmpIndex[0] > j:
                            res.add((nums[i], nums[j], nums[tmpIndex[0]], nums[tmpIndex[1]]))
        return [list(i) for i in res]

[xpresslink]

>>> nums = [1, 0, -1, 0, -2, 2]
>>> target = 0
>>> from itertools import combinations as cb
>>> [c for c in cb(nums, 4) if sum(c)==target]
[(1, 0, -1, 0), (1, -1, -2, 2), (0, 0, -2, 2)]
>>>

[twistoy]

dicti.keys()返回的是一个 list，判断一个元素是不是在一个 list 里是 O(n)的。这里直接 if exp in dicti 就可以了，判断一个元素是不是在一个 dict 里是 O(1)的。

[20015jjw]

建议 lz 搞清楚基本数据结构再写