点分治
跟路径有关的立马想到了点分治
自然我们需要统计每条路径的答案,根据点分治的过程,考虑每条路径,我们希望当前路径上的点作为最小值
那么我们用树状数组保存之前每个最小值对应的最长长度,跑两遍即可
#include<bits/stdc++.h> using namespace std; const int N = 5e4 + 5; vector<int> G[N]; int n, root, m; long long ans; int t[N << 1], vis[N], v[N], mx[N], size[N]; int getsize(int u, int last) { int ret = 1; for(int i = 0; i < G[u].size(); ++i) { int v = G[u][i]; if(vis[v] || v == last) { continue; } ret += getsize(v, u); } return ret; } void getroot(int u, int last, int S) { mx[u] = 0; size[u] = 1; for(int i = 0; i < G[u].size(); ++i) { int v = G[u][i]; if(v == last || vis[v]) { continue; } getroot(v, u, S); mx[u] = max(mx[u], size[v]); size[u] += size[v]; } mx[u] = max(mx[u], S - size[u]); if(mx[u] < mx[root]) { root = u; } } void update(int x, int d) { for(; x; x -= x & -x) { if(d == -1) { t[x] = 0; } else { t[x] = max(t[x], d); } } } int query(int x) { int ret = 0; for(; x <= m; x += x & -x) { ret = max(ret, t[x]); } return ret; } void dp(int u, int last, int mn, int len) { mn = min(mn, v[u]); ans = max(ans, (long long)(len + query(mn + 1)) * mn); for(int i = 0; i < G[u].size(); ++i) { int v = G[u][i]; if(vis[v] || v == last) { continue; } dp(v, u, mn, len + 1); } } void dfs(int u, int last, int mn, int len, int mp) { mn = min(mn, v[u]); update(mn + 1, min(len + 1, mp)); for(int i = 0; i < G[u].size(); ++i) { int v = G[u][i]; if(vis[v] || v == last) { continue; } dfs(v, u, mn, len + 1, mp); } } void solve(int u) { vis[u] = 1; update(v[u] + 1, 1); for(int i = 0; i < G[u].size(); ++i) { int V = G[u][i]; if(vis[V]) { continue; } dp(V, u, v[u], 1); dfs(V, u, v[u], 1, 1e9); } for(int i = 0; i < G[u].size(); ++i) { int V = G[u][i]; if(vis[V]) { continue; } dfs(V, u, v[u], 1, -1); } update(v[u], 1); for(int i = G[u].size() - 1; i >= 0; --i) { int V = G[u][i]; if(vis[V]) { continue; } dp(V, u, v[u], 1); dfs(V, u, v[u], 1, 1e9); } update(v[u], -1); for(int i = G[u].size() - 1; i >= 0; --i) { int V = G[u][i]; if(vis[V]) { continue; } dfs(V, u, v[u], 1, -1); } for(int i = 0; i < G[u].size(); ++i) { int v = G[u][i]; if(!vis[v]) { root = 0; getroot(v, u, getsize(v, u)); solve(root); } } } int main() { scanf("%d", &n); for(int i = 1; i <= n; ++i) { scanf("%d", &v[i]); m = max(m, v[i] + 1); } for(int i = 1; i < n; ++i) { int u, v; scanf("%d%d", &u, &v); G[u].push_back(v); G[v].push_back(u); } mx[0] = 1e9; getroot(1, 0, getsize(1, 0)); solve(root); printf("%lld\n", ans); return 0; }