CodeForces 160D - Distance in Tree 树型DP

题目给了512MB的空间....用dp[k][i]代表以k为起点...往下面走(走直的不打岔)i步能有多少方案....在更新dp[k][i]过程中同时统计答案..

Program:

#include<iostream>
#include<queue>
#include<stack>
#include<stdio.h>
#include<string.h>
#include<algorithm>
#include<cmath>
#define ll long long
#define oo 1000000007
#define MAXN 50005
using namespace std;
struct node
{
      int x,y,next;
}line[MAXN*2];
int n,K,dp[MAXN][502],_next[MAXN];
bool used[MAXN];
ll ans;
void addline(int x,int y,int m)
{
      line[m].next=_next[x],_next[x]=m;
      line[m].x=x,line[m].y=y;
      return;
}
void dfs(int x)
{
      int i,j,k;
      k=_next[x];
      dp[x][0]=1;
      while (k)
      {
            if (!used[line[k].y])
            {
                    used[line[k].y]=true;
                    dfs(line[k].y);
                    for (i=K;i>=1;i--)  ans+=dp[x][K-i]*dp[line[k].y][i-1];
                    for (i=K;i>=1;i--)  dp[x][i]+=dp[line[k].y][i-1];
            }
            k=line[k].next;
      }
      return;
}
int main()
{
      int i,j;
      while (~scanf("%d%d",&n,&K))
      {
             memset(_next,0,sizeof(_next));
             for (i=1;i<n;i++)
             {
                   int x,y;
                   scanf("%d%d",&x,&y);
                   addline(x,y,i*2-1);
                   addline(y,x,i*2);
             }
             memset(used,false,sizeof(used));
             memset(dp,0,sizeof(dp));
             used[1]=true;
             ans=0;
             dfs(1);
             printf("%I64d\n",ans);
      }
      return 0;
}