每天一算：Sort Colors

 1// 三路快速排序的思想
 2// 对整个数组只遍历了一遍
 3// 时间复杂度: O(n)
 4// 空间复杂度: O(1)
 5class Solution {
 6public:
 7    void sortColors(vector<int> &nums) {
 8
 9        int zero = -1;          // [0...zero] == 0
10        int two = nums.size();  // [two...n-1] == 2
11        for(int i = 0 ; i < two ; ){
12            if(nums[i] == 1){
13                 i ++;
14            }else if (nums[i] == 2){
15                 two--;
16                 swap( nums[i] , nums[two]);
17            }else{ // nums[i] == 0
18                 zero++;
19                 swap(nums[zero] , nums[i]);
20                 i++;
21            }
22        }
23    }
24};