Leetcode：Sort Colors

做了这题的感想就是：多思考，其实办法会有很多的。

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?

比较正式的解法：

public class Solution {    public void sortColors(int[] a) {        if(a == null || a.length <= 1)            return;                int pl = 0;        int pr = a.length - 1;        int i = 0;        while(i <= pr){            if(a[i] == 0){                swap(a, pl, i);                pl++;                i++;            }else if(a[i] == 1){                i++;            }else{                swap(a, pr, i);                pr--;            }        }    }        private void swap(int[] a, int i, int j){        int tmp = a[i];        a[i] = a[j];        a[j] = tmp;    }}

在给出网友的一种解法，感觉有点“旁门左道”，但是能很好的解决问题，确实是个好方法。

  public void sortColors(int[] A) {    int i=-1, j=-1, k=-1;    for(int p = 0; p < A.length; p++)    {        if(A[p] == 0)        {            A[++k]=2;            A[++j]=1;            A[++i]=0;        }        else if (A[p] == 1)        {            A[++k]=2;            A[++j]=1;        }        else if (A[p] == 2)        {            A[++k]=2;        }    }}