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# 1. Two Sum

<https://leetcode.com/problems/two-sum/>

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].
```

**Solution:**

```
class Solution(object):
    def twoSum(self, nums, target):
        """
        :type nums: List[int]
        :type target: int
        :rtype: List[int]
        """
        record = {} # to record if show before
        for i in range(len(nums)):
            if target-nums[i] in record: # if show before
                return [record[target-nums[i]], i]
            else:
                record[nums[i]] = i
```