283. Move Zeroes
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.
To do in-place replacement method, you need to use two pointers, one of which keeps sweeping backwards to find a non-zero position, and then swap positions with the previous pointer.