'commit'

TangDou/AcWing/CheckCircle/HDU3342【BFS判环】.cpp

@@ -0,0 +1,40 @@
+#include <bits/stdc++.h>
+using namespace std;
+const int N = 1e3 + 10, M = N << 1;
+int ind[N];
+int n, m;
+//邻接表
+int e[M], h[N], idx, ne[M];
+void add(int a, int b) {
+    e[idx] = b, ne[idx] = h[a], h[a] = idx++;
+}
+int q[N];
+int main() {
+    while (~scanf("%d %d", &n, &m) && n) {
+        memset(ind, 0, sizeof(ind));
+        memset(h, -1, sizeof h);
+        idx = 0;
+
+        for (int i = 1; i <= m; i++) {
+            int a, b;
+            scanf("%d %d", &a, &b);
+            add(a, b);
+            ind[b]++; //记录入度
+        }
+        int hh = 0, tt = -1;
+        for (int i = 0; i < n; i++)
+            if (ind[i] == 0) q[++tt] = i; //入度为0的入队列
+
+        while (hh <= tt) {
+            int u = q[hh++];
+            for (int i = h[u]; ~i; i = ne[i]) {
+                int j = e[i];
+                ind[j]--;                     //拓扑模拟减去边，入度-1
+                if (ind[j] == 0) q[++tt] = j; //如果入度为0，则此节点入队列
+            }
+        }
+        //检查入过队列的节点个数是不是n个，相等则无环
+        hh == n ? puts("YES") : puts("NO");
+    }
+    return 0;
+}