Sort Colors

Description

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

class Solution {
public:
    void sortColors(vector<int>& nums) {
        // all numbers before nums[l] are 0
        // all numbers after nums[r] are 2
        int l = 0, r = nums.size() - 1;
        int i = 0;
        while(i <= r){
            if(nums[i] == 1) ++i;
            else if(nums[i] == 0){
                //swap(nums[l++], nums[i++]);
                nums[i++] = nums[l];
                nums[l++] = 0;
            }else{
                //swap(nums[r--], nums[i]);
                nums[i] = nums[r];
                nums[r--] = 2;
            }
        }
    }
};