Move Zeroes

https://leetcode.com/problems/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.

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

class Solution:

    # 无法改变参数，必须重新赋值，why?
    def moveZeros(self,nums):
        n = len(nums)
        res = [ x for x in nums if x>0 ]

        for i in range(n-len(res)):
            res.append(0)

        for i in xrange(len(nums)):
            nums[i] = res[i]

    def moveZeros2(self,nums):
        if nums == None or len(nums)==0:
            return
        j = 0
        for i in xrange(len(nums)):
            if nums[i]:
                nums[i],nums[j] = nums[j],nums[i]
                j += 1