37.Sort Colors（颜色排序）

Level：

  Medium

题目描述：

Given an array with n objects colored red, white or blue, sort them in-place 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.

Example:

Input: [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]

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 a one-pass algorithm using only constant space?

思路分析：

  三路快排的思想，以1作为标志，比1小的放在一边，比1大的放在另一边一边，但是要注意的一点是大的元素交换后由于该元素没有和标志1作比较，因此需要i=i-1。

代码：

public class Solution{
    public void sortColors(int []nums){
        if(nums==null||nums.length==0)
            return;
        int left=0;
        int right=nums.length-1;
        for(int i=0;i<=right;i++){
            if(nums[i]<1){
                nums[i]=nums[left];
                nums[left]=0;
                left++;
            }else if(nums[i]>1){
                nums[i]=nums[right];
                nums[right]=2;
                right--;
                i=i-1;//交换过来的元素没有和1比较过，所以i减一
            }
        }
    }
}