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].

My first version:

nested two for loops, find the two value sum == target

My second version:

also two loops: the first loop is the same, but the second loop starts from x+1, no need to start index 0.

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