Description
Please judge whether there is a cycle in the directed graph with n vertices and m edges. The parameter is two int arrays. There is a directed edge from start[i] to end[i].

2 <= n <= 10^5
1 <= m <= 4*10^5
1 <= start[i], end[i] <= n
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Example
Given start = [1],end = [2], return “False”。

Explanation:
There is only one edge 1->2, and do not form a cycle.

思路：構建圖結構，記錄每個頂點的入度，若有後向邊存在（拓撲結構中後出現的點有指向已出現的點的邊），則有環。、

class Solution {
public:
    /**
     * @param start: The start points set
     * @param end: The end points set
     * @return: Return if the graph is cyclic
     */
    bool
 
 isCyclicGraph(vector<int> &start, vector<int> &end) {

        int n = 0;
        for(int s:start) n = max(n, s);
        for(int e:end) n = max(n, e);
        vector<int> graph[n+1];
        vector<int> d(n+1); // 保留各頂點的入度,判斷拓撲排序後是否有邊存在
        for(int i=0; i<start.size(); ++i){
            graph[start[i]].push_back(end[i]);
            d[end[i]]++;
        }
        queue
 
<int> que;
        for(int i=1; i<=n; ++i)
            if(graph[i].size() && !d[i]) que.push(i);
        while(!que.empty()){
            int v = que.front(); que.pop();
            for(int edge:graph[v]){
                d[edge]--;
                if(d[edge]==0) que.push(edge);
            }           
        }        
        for(int i=1; i<=n; ++i)
            if(d[i]) return true;
        return false;
    }
};