[LeetCode 解题报告]207. Course Schedule

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?

Example 1:

Input: 2, [[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: 2, [[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.

Note:

1. The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.
2. You may assume that there are no duplicate edges in the input prerequisites.

考察：图，拓扑排序； 

class Solution {
public:
    bool canFinish(int numCourses, vector<vector<int>>& prerequisites){
        vector<vector<int>> graph(numCourses, vector<int>(0));
        vector<int> in(numCourses, 0);
        for (int i = 0; i < prerequisites.size(); i ++) {
            graph[prerequisites[i][1]].push_back(prerequisites[i][0]);
            in[prerequisites[i][0]]++;
        }
        
        queue<int> que;
        for (int i = 0; i < numCourses; i ++) {
            if (in[i] == 0)
                que.push(i);
        }
        
        while(!que.empty()) {
            int cur = que.front();
            que.pop();
            for (int j = 0; j < graph[cur].size(); j ++) {
                in[graph[cur][j]] --;
                if (in[graph[cur][j]] == 0)
                    que.push(graph[cur][j]);
            }
        }
        
        for (int i = 0; i < numCourses; i ++) {
            if (in[i] != 0)
                return false;
        }
        return true;
    }
};