'commit'

TangDou/CSP-J/J-2023/HDU3342.cpp

@@ -0,0 +1,52 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+const int N = 110, M = N << 1;
+int in[N]; // 入度
+// 链式前向星
+int e[M], h[N], idx, w[M], ne[M];
+void add(int a, int b, int c = 0) {
+    e[idx] = b, ne[idx] = h[a], w[idx] = c, h[a] = idx++;
+}
+
+bool solve(int n) {
+    queue<int> q;
+    for (int i = 0; i < n; i++)
+        if (in[i] == 0) q.push(i);
+
+    int cnt = 0;
+    while (q.size()) {
+        int u = q.front();
+        q.pop();
+
+        ++cnt;
+        for (int i = h[u]; ~i; i = ne[i]) {
+            int j = e[i];
+            in[j]--;
+            if (in[j] == 0) q.push(j);
+        }
+    }
+    return cnt == n;
+}
+
+int main() {
+    // 加快读入
+    ios::sync_with_stdio(false), cin.tie(0);
+    int n, m;
+    while (cin >> n >> m && n) {
+        // 多组测试数据，一定要注意两个初始语句！
+        memset(h, -1, sizeof h);
+        idx = 0;
+        memset(in, 0, sizeof in);
+
+        while (m--) {
+            int u, v;
+            cin >> u >> v;
+            in[v]++;
+            add(u, v); // 建正图
+        }
+        bool ans = solve(n);
+        puts(ans ? "YES" : "NO");
+    }
+    return 0;
+}