树哈希与换根dp：CF763D

采用的树哈希函数是：

$$d{p}_x=w_x\times \sum _{y\in x}d{p}_y^2+w_x^2$$

发现从 $x$ 到 $y$ 时只有 $x$ 与 $y$ 的哈希值会变化，分别维护即可

#include<bits/stdc++.h>
using namespace std;
#define int long long
inline int read(){int x=0,f=1;char ch=getchar(); while(ch<'0'||
ch>'9'){if(ch=='-')f=-1;ch=getchar();}while(ch>='0'&&ch<='9'){
x=(x<<1)+(x<<3)+(ch^48);ch=getchar();}return x*f;}
#define Z(x) (x)*(x)
#define pb push_back
//mt19937 rand(time(0));
//mt19937_64 rand(time(0));
//srand(time(0));
#define N 100010
//#define M
#define mo (int)(1e9+123)
int n, m, i, j, k, T;
int pos, mx, cnt, h[N], w[N], dp[N], f[N], u, v; 
map<int, int>mp; 
vector<int>G[N]; int Mod(int a) {return (a%mo+mo)%mo; 
}void add(int x, int k) {mp[x]+=k; if(mp[x]==1 && k==1) ++cnt; if(mp[x]==0 && k==-1) --cnt; 
//	printf("# %lld (%lld): %lld\n", x, mp[x], cnt); 
}void dfs1(int x, int fa) {
//	int s1, s2=0; w[x]=1; for(int y : G[x]) {if(y==fa) continue; dfs1(y, x); w[x]+=w[y]; f[x]=Mod(f[x]+dp[y]*dp[y]); }dp[x]=Mod(w[x]*f[x]%mo+w[x]*w[x]%mo); add(dp[x], 1); 
//	printf("%lld : %lld\n", x, dp[x]); 
}void dfs2(int x, int fa) {int xw, xf, xdp, yw, yf, ydp; for(int y : G[x]) {if(y==fa) continue; 
//		printf("del [%lld] : %lld\n", x, dp[x]); add(dp[x], -1); xdp=dp[x]; xw=w[x]; xf=f[x]; w[x]=w[x]-w[y]; f[x]=Mod(f[x]-dp[y]*dp[y]); dp[x]=(w[x]*f[x]%mo+w[x]*w[x]%mo); 
//		printf("ins [%lld] %lld : %lld\n", x, w[x], dp[x]); add(dp[x], 1); //		printf("del [%lld] : %lld\n", y, dp[y]); add(dp[y], -1); ydp=dp[y]; yw=w[y]; yf=f[y]; w[y]=n; f[y]=Mod(f[y]+dp[x]*dp[x])%mo; dp[y]=(w[y]*f[y]%mo+w[y]*w[y]%mo); //		printf("ins [%lld] : %lld\n", y, dp[y]); add(dp[y], 1); //		printf("%lld : %lld\n", y, cnt); 
//		for(auto t=mp.begin(); t!=mp.end(); ++t) printf("%lld ", t); if(cnt>mx) mx=cnt, pos=y; dfs2(y, x); add(dp[x], -1); add(dp[y], -1); dp[x]=xdp; w[x]=xw; f[x]=xf; add(dp[x], 1); dp[y]=ydp; w[y]=yw; f[y]=yf; add(dp[y], 1); }
}signed main()
{
//	freopen("in.txt", "r", stdin);
//	freopen("out.txt", "w", stdout);
//	T=read();
//	while(T--) {
//
//	}n=read(); for(i=1, k=1; i<n; ++i) {u=read(); v=read(); G[u].pb(v); G[v].pb(u); } dfs1(1, 0); mx=cnt; pos=1; dfs2(1, 0); printf("%lld", pos); return 0;
}