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.

Example:

Input: [0,1,0,3,12]

Output: [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.

---

这道题是的时间复杂度是O(N), 想法很直接。双指针，一个指针用来循环，另一个指针就是指向需要换位的地方。即刚开始没有遇到零的时候，就是和循环指针相同，一旦遇到零了以后，这个指针就是记录着最开始的零的位置，遇到非零的循环值和它交换就行了。这样做也保证了操作次数的最少。

class Solution:
    def moveZeroes(self, nums: List[int]) -> None:
        """
        Do not return anything, modify nums in-place instead.
        """
        if not nums: return None
        zero = 0
        for i in range(len(nums)):
            if nums[i] != 0:
                nums[i], nums[zero] = nums[zero], nums[i]
                zero += 1