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

要求不改变顺讯，明显地，不能使用两根指针进行交换，交换会破坏顺序。

很明显的一种想法是求出零的个数，再逐个把非零的数搬到合法的索引上，但是这样是两次遍历，而且是没有必要的。

只需要一次遍历就可以了，维护一根指针，作为非零元素的索引，当i扫描到末尾时，这个索引应该在第一个零的位置。

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