[LeetCode] 1. Two Sum 两数之和

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：

class Solution:
    def twoSum(self, nums: List[int], target: int) -> List[int]:
        if nums == []:
            return None
        seen = {}
        
        for index,value in enumerate(nums):
            x = target - value
            if x in seen:
                return [seen[x],index]
            else:
                seen[value] = index


#EG: target = 7 
#      list = [1,2,3,4,5,]

#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