【刷题-LeetCode】207. Course Schedule

207. Course Schedule

There are a total of numCourses courses you have to take, labeled from 0 to numCourses-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, is it possible for you to finish all courses?

Example 1:

Input: numCourses = 2, prerequisites = [[1,0]]
Output: true
Explanation: There are a total of 2 courses to take. 
             To take course 1 you should have finished course 0. So it is possible.

Example 2:

Input: numCourses = 2, prerequisites = [[1,0],[0,1]]
Output: false
Explanation: There are a total of 2 courses to take. 
             To take course 1 you should have finished course 0, and to take course 0 you should
             also have finished course 1. So it is impossible.

Constraints:

• The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.
• You may assume that there are no duplicate edges in the input prerequisites.
• 1 <= numCourses <= 10^5

解法1 拓扑排序。如果能完整排序，说明可以修完所有的课程

class Solution {
public:
    bool canFinish(int numCourses, vector<vector<int>>& prerequisites) {
        vector<vector<int>>g(numCourses);
        vector<int>d(numCourses, 0);
        build_graph(numCourses, prerequisites, g, d);
        
        return topological_sort(numCourses, g, d);
    }
    void build_graph(int numCourses, vector<vector<int>>& prerequisites,
                    vector<vector<int>>&g, vector<int>&d){
        for(auto x: prerequisites){
            g[x[1]].push_back(x[0]);
            d[x[0]]++;
        }
    }
    bool topological_sort(int numCourses, vector<vector<int>> &g, vector<int>&d){
        
        vector<bool>vis(numCourses, false);
        bool flag;
        while(1){
            int u = -1;
            flag = false;
            for(int i = 0; i < numCourses; ++i){
                if(d[i] == 0 && vis[i] == false)u = i;
                if(d[i] != 0)flag = true;
            }
            if(u == -1)break;
            vis[u] = true;
            for(auto v : g[u])d[v]--;
        }
        return !flag;
    }
};

解法2 dfs。枚举每个节点，看该节点能不能形成环

class Solution {
public:
    bool canFinish(int numCourses, vector<vector<int>>& prerequisites) {
        vector<vector<int>>g(numCourses);
        vector<int>d(numCourses, 0);
        build_graph(numCourses, prerequisites, g, d);
        
        vector<bool>tested(numCourses, false); // 避免对同一条路径上的节点重复测试
        for(int s = 0; s < numCourses; ++s){
            if(!tested[s]){
                vector<bool>vis(numCourses, false);
                bool flag = false;
                dfs(g, s, vis, tested, flag);
                if(flag)return false;
            }
        }
        return true;
    }
    void dfs(vector<vector<int>> &g, int s, vector<bool>&vis, vector<bool> &tested, bool &flag){
        if(flag)return;
        for(auto v: g[s]){
            if(vis[v]){
                flag = true;
                return;
            }
            vis[v] = true;
            dfs(g, v, vis, tested, flag);
            vis[v] = false;
        }
        tested[s] = true;
    }
    void build_graph(int numCourses, vector<vector<int>>& prerequisites,
                    vector<vector<int>>&g, vector<int>&d){
        for(auto x: prerequisites){
            g[x[1]].push_back(x[0]);
            d[x[0]]++;
        }
    }
};