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.

For example, given nums = [0, 1, 0, 3, 12], after calling your function, nums should be [1, 3, 12, 0, 0].

Note:

1. You must do this in-place without making a copy of the array.
2. Minimize the total number of operations.

思路：顺序遍历数组，每遇到一个非零数字便和前面第一个0进行交换。indexFirstZero记录了第一个0的索引。