算法設計Week4 LeetCode Algorithms Problem #207 Course Schedule

題目要求

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, is it possible for you to finish all courses?

For example:
2, [[1,0]]
There are a total of 2 courses to take. To take course 1 you should have finished course 0. So it is possible.
2, [[1,0],[0,1]]
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.

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解題方案：

class Solution {
public:
    bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {

        // save the graph, make it convenient for search
        vector<unordered_set<int>> matrix(numCourses);
        for(int i = 0; i < prerequisites.size(); i++){
            matrix[prerequisites[i].second].insert(prerequisites[i].first);
        }

        vector<bool> isvisit(numCourses, false);
        vector<bool> visited(numCourses, false);

        for(int i = 0; i < numCourses; i++){
            if(!isvisit[i] && DFS(i, matrix, isvisit, visited)){
                return false;
            }
        }
        return true;
    }

    bool DFS(int course, vector<unordered_set<int>> matrix, vector<bool> isvisit, vector<bool> visited){
        isvisit[course] = true;
        visited[course] = true;
        for(auto it = matrix[course].begin(); it != matrix[course].end(); ++it){
            if(visited[*it] || DFS(*it, matrix, isvisit, visited)){
                return true;
            }
        }
        visited[course] = false;
        return false;
    }
};