CSP 5-6 旅行的期望值

CSP-Training/5/6-journey.cpp

@@ -0,0 +1,53 @@
+//CF893C Journey
+#include <cstdio>
+#include <iostream>
+using namespace std;
+bool visited[100001];
+int head[100001], nxt[200000], to[200000], n, sons[100001];
+double expectation;
+void dfs(int currentPos, double possibility, int depth);
+void insert(int source, int dest, int i);
+int main()
+{
+    scanf("%d", &n);
+    int tempSource, tempDest;
+    for (int i = 0; i < n - 1; i++)
+    {
+        scanf("%d %d", &tempSource, &tempDest);
+        insert(tempSource, tempDest, i * 2 + 1);
+        insert(tempDest, tempSource, i * 2 + 2);
+    }
+    sons[1] /= 2;
+    for (int i = 2; i <= n; i++) //除根节点外，子节点数=连接边数-1
+    {
+        sons[i] = sons[i] / 2 - 1;
+    }
+    dfs(1, 1.0, 0);
+    printf("%.7lf\n", expectation); //HNU CG强制要求7位小数，不然不过
+}
+void dfs(int currentPos, double possibility, int depth)
+{
+    if (visited[currentPos])
+    {
+        return;
+    }
+    if (sons[currentPos] == 0)
+    {
+        expectation += possibility * depth;
+        return;
+    }
+    visited[currentPos] = true;
+    double sonPossibility = possibility / sons[currentPos];
+    for (int i = head[currentPos]; i != 0; i = nxt[i])
+    {
+        dfs(to[i], sonPossibility, depth + 1);
+    }
+}
+void insert(int source, int dest, int i)
+{
+    nxt[i] = head[source];
+    to[i] = dest;
+    head[source] = i;
+    sons[source]++;
+    sons[dest]++;
+}