# Sort Colors

## Description

Given an array with *n* objects colored red, white or blue, sort them [**in-place**](https://en.wikipedia.org/wiki/In-place_algorithm) 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

```cpp
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;
            }
        }
    }
};
```