title

problem link

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?

solution

荷兰国旗包含三种颜色：红、白、蓝。有这三种颜色的球，算法的目标是将这三种球按颜色顺序正确地排列。

它其实是三向切分快速排序的一种变种，在三向切分快速排序中，每次切分都将数组分成三个区间：小于切分元素、等于切分元素、大于切分元素，而该算法是将数组分成三个区间：等于红色、等于白色、等于蓝色。

三向切分

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

也可以

class Solution {
public:
	void sortColors(vector<int>& nums) {
		vector<int> v(3);
		for (auto it : nums) {
			v[it]++;
		}
		int x = 0;
		for (int j = 0; j < 3;j++) {
			for (int i = 0; i < v[j]; i++) {
				nums[x++] = j;
			}
		}
	}
};