Sort Colors - LeetCode

Description

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.

We will use the integers 0, 1, and 2 to represent the color red, white, and blue, respectively.

You must solve this problem without using the library’s sort function.

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]

Constraints:

• n == nums.length
• 1 <= n <= 300
• nums[i] is either 0, 1, or 2.

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

Code

Basic idea 是將沿路遇到的 2 or 0 換到右邊跟左邊，最後剩下在中間的都會是 1。

注意 for 迴圈的終止條件是 i <= two，因為 two 被更新時是向左移動一格，我們並不知道 two 新的位置的值是否需要被 swap，所以要檢查。

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

Source

• Sort Colors - LeetCode