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.
Given nums = [2, 7, 11, 15], target = 9,
Because nums + nums = 2 + 7 = 9,
return [0, 1].
The return format had been changed to zero-based indices. Please read the above updated description carefully.