LeetCode 210. Course Schedule II

题目描述

出处 https://leetcode.com/problems/course-schedule-ii/description/

There are a total of n courses you have to take, labeled from 0 to n-1.

Some courses may have prerequisites, for example to take course 0 you have to first take course 1, which is expressed as a pair: [0,1]

Given the total number of courses and a list of prerequisite pairs, return the ordering of courses you should take to finish all courses.

There may be multiple correct orders, you just need to return one of them. If it is impossible to finish all courses, return an empty array.

Example 1:

Input: 2, [[1,0]]
Output: [0,1]
Explanation: There are a total of 2 courses to take. To take course 1 you should have finished course 0. So the correct course order is [0,1] .

Example 2:

Input: 4, [[1,0],[2,0],[3,1],[3,2]]
Output: [0,1,2,3] or [0,2,1,3]
Explanation: There are a total of 4 courses to take. To take course 3 you should have finished both courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0. So one correct course order is [0,1,2,3]. Another correct ordering is [0,2,1,3] .

---

分析

这道题的解决方法是使用拓扑排序，通过给定的数组构建有向图，通过不断删除入度为0的点和这些点指向的边，完成拓扑排序，最终得到的序列若长度小于最初给定的长度则说明有环，若长度相等则输出对应序列。

---

最终结果

class Solution {
public:
    vector findOrder(int numCourses, vector>& prerequisites) {
        vector> list(numCourses);
        vector degree(numCourses, 0);
        vector result;
        
        for (auto p:prerequisites) {
            list[p.second].push_back(p.first);
            degree[p.first] ++;
        }
        
        queue que;
        for (int i = 0; i < numCourses; i++) {
            if(degree[i] == 0)
                que.push(i);
        }
        
        while(!que.empty()) {
            int num = que.front();
            que.pop();
            result.push_back(num);
            
            for(auto p:list[num]) {
                degree[p]--;
                if (degree[p] == 0)
                    que.push(p);
            }
        }
        
        if (result.size() == numCourses) 
            return result;
        else {
            vector res;
            return res;
        }
    }
};