HDU 3342 Legal or Not

直接判断有无环就行，注意对邻接表初始化，不然就WA了

#include<bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 5;

vector <int> e[maxn << 1];
int in[maxn], n, m, flag, X;
void init() { 
    memset(in, 0, sizeof(in));
    for (int i = 0; i < n; i++) e[i].clear(); // 初始化
    X = 0, flag = false;
}
void topsort() {
    queue <int> q;
    for (int i = 0; i < n; i++) {
        if (!in[i]) q.push(i);
    }
    while (!q.empty()) {
        int u = q.front();
        q.pop();
        X++;
        for (int i = 0; i < e[u].size(); i++) {
            int v = e[u][i];
            in[v]--;
            if (!in[v]) q.push(v);
        }
    }
    if (X == n) flag = true; // 无环则成立
}

int main()
{
    while (cin >> n >> m && n + m) {
        init();
        for (int i = 0; i < m; i++) {
            int ui, vi;
            cin >> ui >> vi;
            e[ui].push_back(vi);
            in[vi]++;
        }
        topsort();
        if (flag) cout << "YES" << endl;
        else cout << "NO" << endl;
    }
    return 0;
}

标签：HDU,3342,vi,++,Legal,int,ui,push,maxn
来源： https://blog.csdn.net/weixin_43851525/article/details/90573303