传送门

class Solution {
public:
    int ans=0;
    vector<vector<int>> edges=vector<vector<int>>(20);
    int zong;
    int m;
    void dfs(int cp, int step)
    {
        if(step==zong)
        {
            if(cp==m-1)
            {
                ans++;
            }
            return;
        }
        for (int np:edges[cp])
        {
            dfs(np,step+1);
        }
    }
    int numWays(int n, vector<vector<int>>& relation, int k) {
        zong=k;
        m=n;
        for (int i=0;i<n;i++)
        {
            edges[relation[i][0]].push_back(relation[i][1]);
        }
        dfs(0,0);
        return ans;
    }
};