hdu 5154 Harry and Magical Computer(拓扑排序)

#include <cstdio>
#include <cstring>
#include <vector>
#include <algorithm>

using namespace std;
const int maxn = 105;

int N, M, s[maxn], v[maxn];
vector<int> g[maxn];

void init () {
int a, b;
memset(s, 0, sizeof(s));
memset(v, 0, sizeof(v));
for (int i = 0; i <= N; i++)
g[i].clear();

for (int i = 0; i < M; i++) {
scanf("%d%d", &a, &b);
g[b].push_back(a);
s[a]++;
}
}

int find () {
for (int i = 1; i <= N; i++) {
if (v[i]) continue;
if (s[i] == 0)
return i;
}
return 0;
}

bool judge () {
for (int i = 0; i < N; i++) {
int u = find();
if (u == 0)
return false;

v[u] = 1;
for (int j = 0; j < g[u].size(); j++)
s[g[u][j]]--;
}
return true;
}

int main () {
while (scanf("%d%d", &N, &M) == 2) {
init();
printf("%s\n", judge() ? "YES" : "NO");
}
return 0;
}