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：
對于整數(shù)排序，若已知各整數(shù)的數(shù)值范圍不大，則考慮用counting sort?。。”绢}只有1-3的取值，因此非常適合。但也如follow up指出，counting sort需要2pass。

思路二：
如何將2pass變?yōu)?pass，重點還是在于只有1-3這3種情況，因此掃描一遍時，遇到0則增加0的數(shù)量并交換到前半段，遇到2則增加2的數(shù)量并交換到后半段，中間剩下的自然是1。

代碼：

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;
    }
    
}