'commit'

TangDou/Topic/P1828.cpp

@@ -5,6 +5,10 @@ const int INF = 0x3f3f3f3f;
 int id[N];
 int a, b, c, g[N][N], res = INF;
 int main() {
+#ifndef ONLINE_JUDGE
+    freopen("P1828_11.in", "r", stdin);
+    // 参考答案：8
+#endif
     // 加快读入
     ios::sync_with_stdio(false), cin.tie(0);
     int p, n, m; // p只奶牛，n个牧场，m条边
@@ -31,7 +35,7 @@ int main() {
     for (int i = 1; i <= n; i++) { // 每个牧场出发
         int ans = 0;
         for (int j = 1; j <= p; j++) ans += g[i][id[j]];
-        res = min(res, ans);
+        if (ans >= 0) res = min(res, ans);
     }
     printf("%d", res);
     return 0;

TangDou/Topic/P1828_11.in

@@ -0,0 +1,9 @@
+3 5 5
+2
+3
+4
+1 2 1
+1 3 5
+2 3 7
+2 4 3
+3 4 5

TangDou/Topic/【Floyd专题】.md

@@ -443,7 +443,51 @@ int main() {
 
 ```
 #### [$P1828$ [$USACO3.2$] 香甜的黄油 $Sweet$ $Butter$](https://www.luogu.com.cn/problem/P1828)
+```cpp {.line-numbers}
+#include <bits/stdc++.h>
+using namespace std;
+const int N = 814;
+const int INF = 0x3f3f3f3f;
+int id[N];
+int a, b, c, g[N][N], res = INF;
+int main() {
+#ifndef ONLINE_JUDGE
+    freopen("P1828_11.in", "r", stdin);
+    // 参考答案：8
+#endif
+    // 加快读入
+    ios::sync_with_stdio(false), cin.tie(0);
+    int p, n, m; // p只奶牛，n个牧场，m条边
+    cin >> p >> n >> m;
 
+    memset(g, 0x3f, sizeof g);
+    for (int i = 1; i <= n; i++) g[i][i] = 0; // 初始化
+
+    for (int i = 1; i <= p; i++) cin >> id[i]; // i号奶牛，在id[i]这个牧场
+
+    while (m--) {
+        cin >> a >> b >> c;
+        g[a][b] = g[b][a] = min(c, g[a][b]);
+    }
+    // 标准的Floyd板子
+    for (int k = 1; k <= n; k++)
+        for (int i = 1; i <= n; i++) {
+            if (g[i][k] == INF) continue; // floyd小优化
+            for (int j = 1; j <= n; j++)
+                if (g[i][j] > g[i][k] + g[k][j])
+                    g[j][i] = g[i][j] = g[i][k] + g[k][j];
+        }
+
+    for (int i = 1; i <= n; i++) { // 每个牧场出发
+        int ans = 0;
+        for (int j = 1; j <= p; j++) ans += g[i][id[j]];
+        if (ans >= 0) res = min(res, ans);
+    }
+    printf("%d", res);
+    return 0;
+}
+
+```
 
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