class Solution {
public:
    struct Node {
        int to, next;
    };
    bool canFinish(int numCourses, vector<pair<int, int>>& prerequisites) {
        if (prerequisites.size() == 0) return 1;
        Node edge[prerequisites.size()];    
        int head[numCourses], que[numCourses], iq = 0, indegree[numCourses];
        memset(head, -1, sizeof(head));
        memset(indegree, 0, sizeof(indegree));
        for (int i = 0; i < prerequisites.size(); i++) {
            edge[i].to = prerequisites[i].first;
            indegree[prerequisites[i].first]++;
            edge[i].next = head[prerequisites[i].second];
            head[prerequisites[i].second] = i;
        }
        for (int i = 0; i < numCourses; i++)
            if (indegree[i] == 0)
                que[iq++] = i;
        for (int i = 0; i < iq; i++) {
            for (int k = head[que[i]]; k != -1; k = edge[k].next) {
                indegree[edge[k].to]--;
                if (indegree[edge[k].to] == 0)
                    que[iq++] = edge[k].to;
            }
        }
        for (int i = 0; i < numCourses; i++)
            if (indegree[i] != 0) return 0;
        return 1;
    }
};