283. 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.

Example:

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

//Time: O(n), Space: O(1)
//注意：这道题关键是想明白如果i, j都指向非0，那么其实是自己和自己交换。
//否则双指针，j用来找非零，i指向当前排好的位置
   public void moveZeroes(int[] nums) {
        if (nums == null || nums.length == 0) {
            return;
        }
        
        int j = 0;
        
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] != 0) {
                swap(i, j, nums);
                j++;
            }
        }
    }
    
    private void swap(int i, int j, int[] nums) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }