Codeforces 161 D. Distance in Tree (树dp)

题目链接：http://codeforces.com/problemset/problem/161/D

题意：

给你一棵树，问你有多少对点的距离为k。

思路：

dp[i][j]表示离i节点距离为j的点个数，2次dfs，一次从底向上，另一次从顶向下。

 //#pragma comment(linker, "/STACK:102400000, 102400000")
 #include <algorithm>
 #include <iostream>
 #include <cstdlib>
 #include <cstring>
 #include <cstdio>
 #include <vector>
 #include <cmath>
 #include <ctime>
 #include <list>
 #include <set>
 #include <map>
 using namespace std;
 typedef long long LL;
 typedef pair <int, int> P;
 const int N = 1e5 + ;
 int dp[][], n, m;
 vector <int> G[];

 void dfs1(int u, int p) {
     dp[u][] = ;
     for(int i = ; i < G[u].size(); ++i) {
         int v = G[u][i];
         if(v == p)
             continue;
         dfs1(v, u);
         for(int i = ; i <= m; ++i) {
             dp[u][i] += dp[v][i - ];
         }
     }
 }

 void dfs2(int u, int p) {
     for(int i = ; i < G[u].size(); ++i) {
         int v = G[u][i];
         if(v == p)
             continue;
         for(int i = m; i >= ; --i) {
             dp[v][i] += dp[u][i - ] - dp[v][i - ];
         }
         dp[v][]++;
         dfs2(v, u);
     }
 }

 int main()
 {
     int u, v;
     while(~scanf("%d %d", &n, &m)) {
         for(int i = ; i < n; ++i) {
             scanf("%d %d", &u, &v);
             G[u].push_back(v);
             G[v].push_back(u);
         }
         if(m == ) {
             printf("%d\n", n - );
         } else {
             dfs1(, -);
             dfs2(, -);
             LL ans = ;
             for(int i = ; i <= n; ++i) {
                 ans += (LL)dp[i][m];
             }
             printf("%lld\n", ans / );
         }
     }
     return ;
 }