CSES – Game Routes

#include<bits/stdc++.h>
#define int long long
#define pii pair<int,int>
#define f first
#define s second
#define fastio ios_base::sync_with_stdio(0);cin.tie(0)
using namespace std;
//declare
const int maxn = 1e5+5, mod = 1e9+7;
int n,m,in[maxn],dp[maxn];
vector<int> G[maxn], ord;
bool vis[maxn];
//
void topo_sort(){
    queue<int> q;
    for(int i=1;i<=n;i++) if(!in[i]) q.push(i);
    while(q.size()){
        int cur = q.front(); q.pop();
        ord.push_back(cur);
        for(int adj: G[cur]){
            if(!--in[adj]) q.push(adj);
        }
    }
}
signed main(){
    fastio;
    cin>>n>>m;
    for(int i=0;i<m;i++){
        int a,b; cin>>a>>b;
        G[a].push_back(b);
        in[b]++;
    }
    topo_sort();
    dp[ord[0]] = 1;
    for(int cur: ord){
        for(int adj: G[cur]){
            (dp[adj] += dp[cur]) %= mod;
        }
    }
    cout<<dp[n]<<"\n";
    return 0;
}