# 075. Sort Colors

## Question 75

<https://leetcode.com/problems/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]
```

### Answer&#x20;

### v1.&#x20;

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.

```python
class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        
        Count0 = Count1 = 0
        for i in nums:
            if i == 0:
                Count0 += 1
            elif i == 1:
                Count1 += 1
        
        Count1 += Count0
        for index in range(0,len(nums)):
            if index < Count0:
                nums[index] = 0
            elif index < Count1:
                nums[index] = 1
            else:
                nums[index] = 2
```

### v2.

Could you come up with a one-pass algorithm using only constant space?

{% tabs %}
{% tab title="for" %}

```python
class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        
        index0, index1, index2 = 0, 0, len(nums)-1
        
        while index1 <= index2:
            if nums[index1] == 0:
                nums[index0], nums[index1] = 0, nums[index0]
                index1 += 1
                index0 += 1
            elif nums[index1] == 2:
                nums[index2], nums[index1] = 2, nums[index2]
                #index1 += 1
                index2 -= 1
            else:
                index1 += 1
            
            #print(index0,index1,index2)
            #print(nums)
```

{% endtab %}

{% tab title="while" %}

```python
class Solution(object):
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        index0, index1 = 0, 0
        
        for i in range(0, len(nums)):
            v = nums[i]
            nums[i] = 2
            
            if v == 0:
                nums[index1] = 1
                index1 += 1
                nums[index0] = 0
                index0 += 1
            elif v == 1:
                nums[index1] = 1
                index1 += 1
```

{% endtab %}
{% endtabs %}


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://quenluo.gitbook.io/leecode-python/075.-sort-colors.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.