001-Two Sum

语言：python3

v1：轮询

    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        for i in range(len(nums)):
            for j in range(i+1, len(nums)):
                if nums[i] + nums[j] == target:
                    return [i,j]

结果：超时了，提交失败

v2：建立字典，循环字典

class Solution:
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        d = {}
        for i in range(len(nums)):
            if not nums[i] in d:
                d[nums[i]] = i   #保存数组位置信息
            if target - nums[i] in d:
                if d[target - nums[i]] < i:  #防止 6-3=3的情况
                    return [d[target - nums[i]], i]

总结：字典映射结构比for循环效率更