2019年5月25日   开始了我的刷题之路，此笔记记录刷题流程

[LeetCode] 1. Two Sum 两数之和

 

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,

return [0, 1]

 

 

思路：遍历一次，遍历过程中建立字典，和减法，除法都有对称性，如果在列表中找到了目标1，根据 sum = a + b  那么后面必然还会找到一个目标2 , 

 

Python 代码：

1 class Solution: 2     def twoSum(self, nums: List[int], target: int) -> List[int]: 3         if nums == []: 4             return None 5         seen = {} 6          7         for index,value in enumerate(nums): 8             x = target - value 9             if x in seen:10                 return [seen[x],index]11             else:12                 seen[value] = index13 14 15 #EG: target = 7 16 #      list = [1,2,3,4,5,]17 18 #如果a能成功，b不在字典里，但是列表里的确有b ，不要慌，这时把a放到字典里，减法有对称性，扫描到b的时候必然能成功找到 a 在字典里，返回两个下标