Sort Colors

来源：Leetcode

原帖：http://oj.leetcode.com/problems/sort-colors/

题目：
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?

Solution: 0 0 0 1 1 1 1 ...... 2 2 2 2
               |         |      |
             zero        i     two
              ->        ->     <-

代码：

 class Solution {  
 public:  
   //Version 1: 从左到右遍历  
   void sortColors(int A[], int n) {  
     int zero = 0, two = n-1;  
     int i = 0;  
     while (i <= two) {  
       switch(A[i]) {  
       case 0:  
         swap(A[i++], A[zero++]);  
         break;  
       case 1:  
         i++;  
         break;  
       case 2:  
         swap(A[i], A[two--]);  
       }  
     }  
   }  
 };  
   
 class Solution {  
 public:  
   //Version 2: partition两次;  
   int partition(int A[], int len, int val) {  
     int i = 0, j = len -1;  
     while (i <= j) {  
       while (i <= j && A[i] <= val) i++;  
       while (i <= j && A[j] > val) j--;  
       if (i < j) swap(A[i++], A[j--]);  
     }  
     return i;  
   }  
     
   void sortColors(int A[], int n) {  
     int start = partition(A, n, 0);  
     partition(A + start, n - start, 1);  
   }  
 };