【codeforces】718D. Andrew and Chemistry 【树同构】

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题目链接：D. Andrew and Chemistry

题意：问加一个叶子后树的方案数，同构的算一种。

树同构模板题，同构的儿子选一个走就好。

#include <bits/stdc++.h>using namespace std ;typedef long long LL ;#define clr( a , x ) memset ( a , x , sizeof a )const int MAXN = 100005 ;const int MAXE = 1000005 ;const int mod = 998244353 ;const LL seed = 9493101 ;const LL M = 1e9 + 9 ;struct Edge {    int v , n ;    Edge () {}    Edge ( int v , int n ) : v ( v ) , n ( n ) {}} ;int cnt ;vector < int > G[MAXN] ;vector < pair < int , int > > H[MAXN] ;int h[MAXN] , p[MAXN] , siz[MAXN] ;int n ;int X , X2 ;int ans ;int dist ;void predfs ( int u , int f , int dis ) {    if ( dis > dist ) {        dist = dis ;        X = u ;    }    for ( int i = 0 ; i < G[u].size () ; ++ i ) {        int v = G[u][i] ;        if ( v == f ) continue ;        p[v] = u ;        predfs ( v , u , dis + 1 ) ;    }}void dfs1 ( int u , int f ) {    H[u].clear () ;    siz[u] = 1 ;    h[u] = 1 ;    int leaf = 1 ;    for ( int i = 0 ; i < G[u].size () ; ++ i ) {        int v = G[u][i] ;        if ( v == f ) continue ;        leaf = 0 ;        dfs1 ( v , u ) ;        siz[u] += siz[v] ;        H[u].push_back ( make_pair ( h[v] , v ) ) ;    }    if ( !leaf ) {        sort ( H[u].begin () , H[u].end () ) ;        int cnt = 1 ;        h[u] = ( 1LL * h[u] * H[u][0].first ) % mod ;        for ( int i = 1 ; i < H[u].size () ; ++ i ) {            h[u] = ( 1LL * h[u] * H[u][i].first ) % mod ;        }    }    h[u] = ( 1LL * h[u] + siz[u] ) % M ;    h[u] = ( 1LL * h[u] * h[u] ) % mod ;}void dfs ( int u , int f , int num ) {    int t = H[u].size () ;    if( num + t < 4 ) ++ ans ;    for ( int i = 0 ; i < t ; ++ i ) {        if ( !i || H[u][i].first != H[u][i - 1].first ) dfs ( H[u][i].second , u , 1 ) ;    }}void solve () {    cnt = 0 ;    for ( int i = 1 ; i <= n ; ++ i ) {        G[i].clear () ;    }    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 ) ;    }    dist = -1 ;    predfs ( 1 , 0 , 0 ) ;    dist = -1 ;    predfs ( X , 0 , 0 ) ;    int f = dist & 1 ;    dist >>= 1 ;    while ( dist -- ) X = p[X] ;    X2 = X ;    if ( f ) X = p[X] ;    dfs1 ( X , X2 ) ;    if ( X != X2 ) dfs1 ( X2 , X ) ;    ans = 0 ;    if ( X == X2 ) {        dfs ( X , X2 , 0 ) ;    } else if ( h[X] == h[X2] ) {        dfs ( X , X2 , 1 ) ;    } else {        dfs ( X , X2 , 1 ) ;        dfs ( X2 , X , 1 ) ;    }    printf ( "%d\n" , ans ) ;}int main () {    while ( ~scanf ( "%d" , &n ) ) solve () ;    return 0 ;}

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