LeetCode 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.

click to show follow up.

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?

方法1：遍歷一遍，計算0、1、2的個數，再遍歷一遍根據個數賦值

方法2：遍歷一遍，如果是0，往前置換；如果是2，往後置換

public class Solution {
    public void sortColors(int[] A) {
        int len=A.length;
        int start=0;
        if(len==0)  //數組爲空的情況
        	return;
        int end=len-1;
        for(int i=0;i<len;i++)
        {
        	while(start<len && A[start]==0)  //防止數組越界
        		start++;
        	if(i<start)
        		i=start;
        	while(end>=0 && A[end]==2)  //防止數組越界
    			end--;
        	if(i>end)
        		break;
        	
        	if(A[i]==2)
        	{
        		
        		A[i]=A[end];
        		A[end]=2;
        		end--;
        	}
        	if(A[i]==0)   //不能用else if，需要考慮置換的數值是否爲0
        	{
        		
        		A[i]=A[start];
        		A[start]=0;
        		start++;
        	}
        	
        }
    }
}