LeetCode 1. Two Sum

题目：   

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.

Example:

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

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

题意：

在数组中返回2个数之和为目标数的下标。

题解：（By python）

1. 两次循环求解

class Solution(object):
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]

此复杂度为O（n^2）。

2. 使用python字典存储方式

    格式----   数值：对应下标

class Solution(object):
def twoSum(self, nums, target):
"""
:type nums: List[int]
:type target: int
:rtype: List[int]
"""
all = {} #定义字典
for item in range(len(nums)):
if target-nums[item] in all:
return [all[target-nums[item]],item]
all[nums[item]]=item  #存入字典

此复杂度为O（n）。