Codeforces Round #382 (Div. 2) E. Ostap and Tree

樹DP 初始化有個小技巧
           

#include <cstdio>
#include <vector>
#include <cstring>
#include <algorithm>
using namespace std;
typedef long long ll;
typedef unsigned int uii;
const ll mod=1e9+7;
int n,k,u,v;
ll dp[105][45],f[45];
bool vis[105];
vector<int> vec[105];
void dfs(int u) {
    vis[u]=true;
    dp[u][0]=dp[u][k+1]=1;
    for (uii t=0;t<vec[u].size();++t) {
        int v=vec[u][t];
        if (vis[v])
            continue;
        dfs(v);
        memset(f,0,sizeof f);
        for (int i=0;i<=2*k;++i)
            for (int j=0;j<=2*k;++j)
                if (i+j<=2*k)
                    f[min(i,j+1)]=(f[min(i,j+1)]+dp[u][i]*dp[v][j])%mod;
                else
                    f[max(i,j+1)]=(f[max(i,j+1)]+dp[u][i]*dp[v][j])%mod;
        memcpy(dp[u],f,sizeof dp[u]);
    }
}
int main()
{
    scanf("%d%d",&n,&k);
    for (int i=0;i<n-1;++i) {
        scanf("%d%d",&u,&v);
        vec[u].push_back(v);
        vec[v].push_back(u);
    }
    dfs(1);
    ll res=0;
    for (int i=0;i<=k;++i)
        res=(res+dp[1][i])%mod;
    printf("%lld\n",res);
    return 0;
}