LeetCode 75 Sort Colors

Given an array with n objects colored red, white or blue, sort them so that objects of the same color are adjacent, with the colors in the order red, white and blue.
Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.
Note:
You are not suppose to use the library's sort function for this problem.

Follow up:
A rather straight forward solution is a two-pass algorithm using counting sort.First, iterate the array counting number of 0's, 1's, and 2's, then overwrite array with total number of 0's, then 1's and followed by 2's.
Could you come up with an one-pass algorithm using only constant space?

思路一：
对于整数排序，若已知各整数的数值范围不大，则考虑用counting sort！！！本题只有1-3的取值，因此非常适合。但也如follow up指出，counting sort需要2pass。

思路二：
如何将2pass变为1pass，重点还是在于只有1-3这3种情况，因此扫描一遍时，遇到0则增加0的数量并交换到前半段，遇到2则增加2的数量并交换到后半段，中间剩下的自然是1。

代码：

public class Solution {
    public void sortColors(int[] nums) {
        int lastzero = 0, firsttwo = nums.length-1;
        for (int i = 0; i <= firsttwo; i++) {
            if (nums[i] == 0) swap(nums,i,lastzero++);
            else if (nums[i] == 2) **swap(nums,i--,firsttwo--);**
        }
    }
    
    public void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
    
}