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.

 Notice

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

Example

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

分析：

用一个指针 p 指向第一个元素，从第一个元素开始，如果当前元素nums[i]不是0，那么就和nums[p]互换位置，然后p++. 这里千万不能从尾部开始，否则不能保证order一致。

public class Solution {
    /**
     * @param nums an integer array
     * @return nothing, do this in-place
     */
    public void moveZeroes(int[] nums) {
        if (nums == null || nums.length == 0) return;
        int start = 0;
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] != 0) {
                swap(nums, i, start);
                start++;
            } 
        }
    }
    
    public void swap(int[] arr, int idx1, int idx2) {
        int temp = arr[idx1];
        arr[idx1] = arr[idx2];
        arr[idx2] = temp;
    }
}