This problem was asked by Facebook.
Given an array nums, write a function to move all 0's to the end of it while maintaining the relative order of the non-zero elements.
You must do this in-place without making a copy of the array.
Minimize the total number of operations.
Input: [0, 1, 0, 3, 12]
Output: [1, 3, 12, 0, 0]