CF1292C Xenon‘s Attack on the Gangs

https://www.luogu.com.cn/problem/CF1292C

考虑从小到大放数
发现只有一条链的会产生贡献（放的数会连成一条链）

然后DP即可

#include<bits/stdc++.h>
#define N 4005
#define ll long long
using namespace std;
struct edge {int nxt, v;
} e[N << 1];
int p[N], eid;
void init() {memset(p, -1, sizeof p);eid = 0;
}
void insert(int u, int v) {e[eid].v = v;e[eid].nxt = p[u];p[u] = eid ++;
}
int size[N][N], fa[N][N], n;
ll f[N][N];
void dfs(int u, int rt) {size[rt][u] = 1;for(int i = p[u]; i + 1; i = e[i].nxt) {int v = e[i].v;if(v == fa[rt][u]) continue;fa[rt][v] = u; dfs(v, rt); size[rt][u] += size[rt][v];}
}
ll dp(int u, int v) {// printf("%d %d\n", u, v);if(u == v) return 0;if(f[u][v]) return f[u][v];return f[u][v] = max(dp(u, fa[u][v]), dp(v, fa[v][u])) + 1ll * size[u][v] * size[v][u];
}
int main() {init();scanf("%d", &n);for(int i = 1; i < n; i ++) {int u, v;scanf("%d%d", &u, &v);insert(u, v);insert(v, u);}for(int i = 1; i <= n; i ++) dfs(i, i);ll ans = 0;for(int i = 1; i <= n; i ++)for(int j = 1; j <= n; j ++) ans = max(ans, dp(i, j));printf("%lld", ans);return 0;
}

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