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:

nums = [2, 7, 11, 15]

target = 9

returns

[0, 1]

because nums[0] + nums[1] = 2 + 7 = 9.

UPDATE (2016/2/13): The return format had been changed to zero-based indices. Please read the above updated description carefully.