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53 lines
1.2 KiB
53 lines
1.2 KiB
#include <bits/stdc++.h>
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using namespace std;
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const int N = 110, M = N << 1;
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int in[N]; // 入度
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// 链式前向星
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int e[M], h[N], idx, w[M], ne[M];
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void add(int a, int b, int c = 0) {
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e[idx] = b, ne[idx] = h[a], w[idx] = c, h[a] = idx++;
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}
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bool solve(int n) {
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queue<int> q;
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for (int i = 0; i < n; i++)
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if (in[i] == 0) q.push(i);
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int cnt = 0;
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while (q.size()) {
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int u = q.front();
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q.pop();
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++cnt;
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for (int i = h[u]; ~i; i = ne[i]) {
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int j = e[i];
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in[j]--;
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if (in[j] == 0) q.push(j);
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}
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}
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return cnt == n;
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}
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int main() {
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// 加快读入
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ios::sync_with_stdio(false), cin.tie(0);
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int n, m;
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while (cin >> n >> m && n) {
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// 多组测试数据，一定要注意两个初始语句！
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memset(h, -1, sizeof h);
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idx = 0;
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memset(in, 0, sizeof in);
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while (m--) {
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int u, v;
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cin >> u >> v;
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in[v]++;
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add(u, v); // 建正图
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}
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bool ans = solve(n);
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puts(ans ? "YES" : "NO");
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}
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return 0;
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}
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