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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].
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给定一个整数数组，返回两个元素的索引，使得这两个元素相加的值等于一个给定的target。

您可以假设每个输入都恰有一个解决方案。

例：
给定nums = [2,7,11,15]，target = 9，

因为nums [0] + nums [1] = 2 + 7 = 9，
return [0，1]。
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 1 class Solution(object):
 2     def twoSum(self, nums, target):
 3         """
 4         :type nums: List[int]
 5         :type target: int
 6         :rtype: List[int]
 7         """
 8         arr = {}                       # 定义一个空字典
 9         length = len(nums)
10         for i in range(length):
11             if (target - nums[i]) in arr:     # 如果target减去当前的元素的值存在于空字典中时，符合题意。
12                 return [arr[target - nums[i]], i]  # 返回target-nums[i]和i的索引
13             arr[nums[i]] = i                   # 否则将该元素加入字典arr
14 
15 # 该方法accepted了，不知道是否有更优的解法
16 a = Solution().twoSum([3, 2, 4], 6)
17 print(a)