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].