leetcode 75. Sort Colors （荷兰三色旗问题）

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?

 

class Solution {
public:
    void sortColors(vector<int>& nums) {
        int i = 0, j = 0, k = nums.size() - 1;
        while (j <= k) {
            if (nums[j] == 0) { 
                swap(nums[i++], nums[j++]); 
            } else if (nums[j] == 2) { 
                swap(nums[k--], nums[j]); 
            } else { 
                j++; 
            }
        }
    }
};