mirror of
https://github.com/Kiritow/OJ-Problems-Source.git
synced 2024-03-22 13:11:29 +08:00
92 lines
3.0 KiB
C++
92 lines
3.0 KiB
C++
#include <iostream>
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#include <cstdio>
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#include <cstring>
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#include <algorithm>
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#include <queue>
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#include <climits>
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#include <ctime>
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#include <cmath>
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#include <set>
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#include <map>
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#include <string>
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#define MAX(a,b) ((a)>(b)?(a):(b))
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#define MIN(a,b) ((a)<(b)?(a):(b))
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#define abs(x) ((x)>0?(x):(-(x)))
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#define FOR(i,a,b) for(int i = (a);i<=(b);i++)
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#define FORD(i,a,b) for(int i = (a);i>=(b);i--)
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#define REP(i,n) for(int i = 0;i<(n);i++)
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#define rst(x,k) memset(x,k,sizeof(x))
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#define lowbit(x) ((x)&(-(x)))
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#define h(x) (1<<(x))
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#define lson (ind<<1)
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#define rson (ind<<1|1)
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#define eps 1e-6
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#define maxn 480
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#define INF 1000000000
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#define mod 1000000007LL
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#define base 160
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#define link fsafsdfas
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using namespace std;
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typedef long long LL;
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struct node{
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int t,nxt;
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}edge[maxn<<1];
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int headline[maxn],E;
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void add(int f,int t){
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edge[E].t = t;
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edge[E].nxt = headline[f];
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headline[f] = E++;
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}
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LL dp[310][3][480];
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LL tt[310][3][480];
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int n;
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int high[310],low[310];
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int up,down;
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void dfs(int u,int fa){
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dp[u][0][base] = 2;
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dp[u][1][base] = 1;
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high[u] = low[u] = 0;
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for(int i = headline[u];~i;i = edge[i].nxt){
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int v = edge[i].t; if(v == fa)continue;
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dfs(v,u);rst(tt[u],0);
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FOR(p,low[u],high[u]){
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FOR(q,low[v]-1,high[v]+1){
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if((p+q > n) || (p+q < (-n)/2-2))continue;
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tt[u][0][p+q+base] += dp[u][0][p+base]*dp[v][0][q+base] + dp[u][0][p+base]*dp[v][1][q+base-1] + dp[u][0][p+base]*dp[v][2][q+base+1];
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tt[u][1][p+q+base] += dp[u][1][p+base]*dp[v][0][q+base] + dp[u][1][p+base]*dp[v][2][q+base] + dp[u][2][p+base]*dp[v][1][q+base];
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tt[u][2][p+q+base] += dp[u][1][p+base]*dp[v][1][q+base] + dp[u][2][p+base]*dp[v][0][q+base] + dp[u][2][p+base]*dp[v][2][q+base];
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tt[u][0][p+q+base] %= mod; if(tt[u][0][p+q+base] && p+q > high[u]) high[u] = p+q; if(tt[u][0][p+q+base] && p+q < low[u])low[u] = p+q;
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tt[u][1][p+q+base] %= mod; if(tt[u][1][p+q+base] && p+q > high[u]) high[u] = p+q; if(tt[u][1][p+q+base] && p+q < low[u])low[u] = p+q;
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tt[u][2][p+q+base] %= mod; if(tt[u][2][p+q+base] && p+q > high[u]) high[u] = p+q; if(tt[u][2][p+q+base] && p+q < low[u])low[u] = p+q;
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}
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}
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FOR(j,low[u],high[u]){
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dp[u][0][j+base] = tt[u][0][j+base];
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dp[u][1][j+base] = tt[u][1][j+base];
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dp[u][2][j+base] = tt[u][2][j+base];
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}
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}
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}
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void solve(){
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rst(headline,-1); E = 0;
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rst(dp,0);
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REP(i,n-1){
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int a,b; scanf("%d%d",&a,&b);
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add(a,b); add(b,a);
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}
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up = n; down = -n/2-1;
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dfs(1,0);
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LL ans = 0;
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FOR(i,-1,up){
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ans += dp[1][0][i+base]*(MAX(i,0))%mod;
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ans += dp[1][1][i+base]*(MAX(i+1,0))%mod;
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ans += dp[1][2][i+base]*(MAX(i-1,0))%mod;
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ans %= mod;
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}
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printf("%I64d\n",ans*3%mod);
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}
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int main(void){
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while(EOF != scanf("%d",&n))solve();
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return 0;
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}
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