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].
简言之，查找给定数组内两个数，使得两数和为给定的target
思路：
遍历给定数组，用一个哈希表存放已经遍历过的值，key是nums中的值，value是其位置索引。
对于当前遍历的nums[i]，查找哈希表中是否存在target-nums[i]，若存在，则返回二者索引，否则将(key=nums[i],value=i)放入哈希表中，继续遍历
时间复杂度：O(n)
python代码如下，已AC:
class Solution(object):
def twoSum(self, nums, target):
"""
:type nums: List[int]
:type target: int
:rtype: List[int]
"""
dictMap = {}
for index, value in enumerate(nums):
if target - value in dictMap:
return [dictMap[target - value] , index ]
dictMap[value] = index
