原题：
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.
翻译：
给出一个数字列表和一个目标值（target），假设列表中有且仅有两个数相加等于目标值，我们要做的就是找到这两个数，并返回他们的索引值。
举例：
Given nums = [2, 7, 11, 15], target = 9, Because nums[0] + nums[1] = 2 + 7 = 9, return [0, 1].
解答：
# 方法1：双层循环，面试不会通过，时间复杂度太高
def twoSum_way1(self,nums,target):
"""
:param nums: 整数列表
:param target:目标值
:return:result 索引值
"""
result = []
for i in range(len(nums)):
for j in range(i+1,len(nums)):
if nums[i] + nums[j] == target:
result.append(i)
result.append(j)
return result
#方法2：通过判断target与某一个元素的差值是否也在列表之中,类似方法1,同样是面试官不期望的回答
def twoSum_way2(self,nums,target):
"""
:param nums: 整数列表
:param target:目标值
:return:result 索引值
"""
result = []
for i in range(len(nums)):
first_num = nums[i]
second_num = target - first_num
if second_num in nums:
j = nums.index(second_num)
if i != j:
result.append(i)
result.append(j)
return result
#适合面试的方法：
#方法3：通过创建字典，将nums里的值和序号对应起来，
# 并创建另一个字典存储目标值（Target）-nums的值，
# 通过判断该值是否在nums内进行判断并返回其对应索引值
class Solution:
def twoSum_way3(self, nums, target):
"""
:param nums: 整数列表
:param target:目标值
:return:result 索引值
"""
#创建第一个字典：用于存储整数列表nums的元素值和对应索引
num_dict = {nums[i]: i for i in range(len(nums))}
#创建第二个字典：存储target-列表中的元素的值
num_dict2 = {i: target - nums[i] for i in range(len(nums))}
#判断num_dict2的值是否是输入列表中的元素，如果是返回索引值，不是则往下进行
result = []
for i in range(len(nums)):
j = num_dict.get(num_dict2.get(i))
if (j is not None) and (j != i):
result = [i,j]
break
return result
#方法4:改进方法3，让代码简洁一些
class Solution:
def twoSum_way4(self, nums, target):
"""
:param nums: 整数列表
:param target:目标值
:return:result 索引值
"""
num_dict = {nums[i]: i for i in range(len(nums))}
for i in range(len(nums) -1):
difference = target - nums[i]
if difference in num_dict and i != num_dict[difference]:
return [i,num_dict[difference]]
return None
终极写法：
class Solution(object):
def twoSum(self, nums, target):
"""
:type nums: List[int]
:type target: int
:rtype: List[int]
"""
dict = {}
for index,num in enumerate(nums):
another_num = target - num
dict[num] = index
if another_num in dict:
return [dict[another_num], index]
return None