拓扑排序

#include <iostream>
#include <cstring>
#include <vector>
#include <queue>
using namespace std;
const int N=1e5+10;

int n, m;
vector<int> G[N];
int indegree[N];

vector<int> ans;

bool topSort() {
    int cnt = 0;
    queue<int> q;
    for(int i=1; i<=n; ++i) {
        if(indegree[i] == 0) {
            q.push(i);
        }
    }

    while(!q.empty()) {
        int u = q.front();q.pop();
        ans.push_back(u);
        for(int i=0; i<G[u].size(); ++i) {
            int v = G[u][i];
            indegree[v] --;
            if(indegree[v] == 0) q.push(v);
        }

        G[u].clear();
        cnt ++;
    }
    return cnt == n;

}

int main() {
    memset(indegree, 0, sizeof indegree);
    cin >> n >> m;

    while(m --) {
        int a,b;
        cin >> a >> b;
        G[a].push_back(b);
        indegree[b] ++;
    }
    if(!topSort())cout << -1 << endl;
    else{
        for(int i=0; i<ans.size(); ++i) {
            cout << ans[i];
            if(i!=ans.size()-1) cout << " ";
        }
        cout << endl;
    }

    return 0;
}