refactor(P8817): 重构题目解法并优化性能

将原递归实现改为BFS算法计算节点间距离
使用预处理和剪枝优化计算效率
添加完整解题逻辑计算最大得分

src/8/15/P8817.cpp

@@ -1,59 +0,0 @@
-#include <cstdint>
-#include <functional>
-#include <iostream>
-#include <queue>
-#include <utility>
-#include <vector>
-
-using ll = int64_t;
-ll n,m,k;
-std::vector<ll> score;
-std::vector<std::vector<ll>> edg;
-std::vector<std::vector<bool>> cango;
-std::vector<std::vector<ll>> best3;
-
-void dfscango(ll from,ll cur, ll curk){
-    if(curk>k){
-        return;
-    }
-    for(ll i=0;i<edg[cur].size();i++){
-        cango[from][edg[cur][i]]=true;
-        dfscango(from, edg[cur][i], curk+1);
-    }
-    return;
-}
-
-int main(){
-    std::cin>>n>>m>>k;
-    score.resize(n+1);
-    edg.resize(n+1);
-    cango.resize(n+1,std::vector<bool>(n+1));
-    best3.resize(n+1);
-    for(ll i=2;i<=n;i++){
-        std::cin>>score[i];
-    }
-    for(ll i=1;i<=m;i++){
-        ll u,v;
-        std::cin>>u>>v;
-        edg[u].push_back(v);
-        edg[v].push_back(u);
-    }
-    for(ll i=1;i<=n;i++){
-        dfscango(i, i, 0);
-    }
-    for(ll i=2;i<=n;i++){
-        std::priority_queue<std::pair<ll, ll>,std::vector<std::pair<ll, ll>>,std::greater<>> pr;
-        for(ll j=2;j<=n;j++){
-            if(i==j)continue;
-            if(cango[i][j]&&cango[j][1]){
-                pr.emplace(score[j],j);
-            }
-        }
-        //TODO
-    }
-    for(ll B=2;B<=n;B++){
-        for(ll C=B+1;C<=n;C++){
-
-        }
-    }
-}

src/9/25/P8817.cpp

@@ -0,0 +1,99 @@
+#include <algorithm>
+#include <iostream>
+#include <queue>
+#include <vector>
+
+using ll = long long;
+
+const int MAXN = 2505;
+
+int n;
+int m;
+int k;
+ll sc[MAXN];
+
+std::vector<int> adj[MAXN];
+int dist[MAXN][MAXN];
+
+std::vector<int> pre[MAXN];
+
+void bfs(int s) {
+    std::queue<std::pair<int, int>> q;
+    q.push({s, 0});
+    dist[s][s] = 0;
+
+    while (!q.empty()) {
+        auto [u, d] = q.front();
+        q.pop();
+        if (d > k) {
+            continue;
+        }
+        for (int v : adj[u]) {
+            if (dist[s][v] == -1) {
+                dist[s][v] = d + 1;
+                q.push({v, d + 1});
+            }
+        }
+    }
+}
+
+int main() {
+    std::ios_base::sync_with_stdio(false);
+    std::cin.tie(nullptr);
+
+    std::cin >> n >> m >> k;
+
+    for (int i = 2; i <= n; ++i) {
+        std::cin >> sc[i];
+    }
+
+    for (int i = 0; i < m; ++i) {
+        int u, v;
+        std::cin >> u >> v;
+        adj[u].push_back(v);
+        adj[v].push_back(u);
+    }
+
+    for (int i = 1; i <= n; ++i) {
+        for (int j = 1; j <= n; ++j) {
+            dist[i][j] = -1;
+        }
+    }
+
+    for (int i = 1; i <= n; ++i) {
+        bfs(i);
+    }
+
+    for (int i = 2; i <= n; ++i) {
+        for (int j = 2; j <= n; ++j) {
+            if (i == j)
+                continue;
+            if (dist[1][j] != -1 && dist[j][i] != -1) {
+                pre[i].push_back(j);
+            }
+        }
+        std::sort(pre[i].begin(), pre[i].end(), [&](int a, int b) { return sc[a] > sc[b]; });
+        if (pre[i].size() > 3) {
+            pre[i].resize(3);
+        }
+    }
+
+    ll ans = 0;
+    for (int b = 2; b <= n; ++b) {
+        for (int c = 2; c <= n; ++c) {
+            if (b == c)
+                continue;
+            if (dist[b][c] != -1) {
+                for (int a : pre[b]) {
+                    for (int d : pre[c]) {
+                        if (a != c && a != d && b != d) {
+                            ans = std::max(ans, sc[a] + sc[b] + sc[c] + sc[d]);
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    std::cout << ans << std::endl;
+}