【HDOJ】5154 Harry and Magical Computer

拓扑排序。

/* 5154 */
#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <queue>
#include <vector>
using namespace std;

#define MAXN 105

vector<int> vec[MAXN];
vector<int>::iterator iter;
int cnt[MAXN];
int n, m;

void init() {
    int i;
    
    memset(cnt, 0, sizeof(cnt));
    for (i=1; i<=n; ++i)
        vec[i].clear();
}

bool topSort() {
    int i, j, k, tmp;
    int c = 0;
    queue<int> Q;
    
    for (i=1; i<=n; ++i) {
        if (cnt[i] == 0) {
            Q.push(i);
        }
    }
    
    while (!Q.empty()) {
        ++c;
        i = Q.front();
        Q.pop();
        for (iter=vec[i].begin(); iter!=vec[i].end(); ++iter) {
            if (--cnt[*iter] == 0) {
                Q.push(*iter);
            }
        }
    }
    
    if (c < n)
        return false;
    else
        return true;
}

int main() {
    int i, j, k;
    bool flag;
    
    #ifndef ONLINE_JUDGE
        freopen("data.in", "r", stdin);
    #endif
    
    while (scanf("%d %d", &n, &m) != EOF) {
        init();
        for (i=0; i<m; ++i) {
            scanf("%d %d", &j, &k);
            vec[j].push_back(k);
            ++cnt[k];
        }
        flag = topSort();
        if (flag)
            printf("YES
");
        else
            printf("NO
");
    }
    
    return 0;
}