75. Sort Colors

https://leetcode.com/problems/sort-colors/

Problem

Given an array nums 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.

Follow up:

• Could you solve this problem without using the library's sort function?

• Could you come up with a one-pass algorithm using only O(1) constant space?

Example 1:

Input: nums = [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]

Example 2:

Input: nums = [2,0,1]
Output: [0,1,2]

Example 3:

Input: nums = [0]
Output: [0]

Example 4:

Input: nums = [1]
Output: [1]

Constraints:

• n == nums.length

• 1 <= n <= 300

• nums[i] is 0, 1, or 2.

Solution

1. Two pointer

class Solution {
public:
    inline void swap(int &a, int &b) {
        int temp = a;
        a = b;
        b = temp;
    }
    void sortColors(vector<int>& nums) {
        int index = 0;
        for (int i = 0; i < nums.size(); ++i) {
            if (nums[i] == 0) swap(nums[i], nums[index++]);
        }
        for (int i = index; i < nums.size(); ++i) {
            if (nums[i] == 1) swap(nums[i], nums[index++]);
        }
    }
};

2. Optimized one pass using two pointer

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

• #twopointer

• #important