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 + 5;
int dp[50005][505], n, m;
vector <int> G[50005];

void dfs1(int u, int p) {
    dp[u][0] = 1;
    for(int i = 0; i < G[u].size(); ++i) {
        int v = G[u][i];
        if(v == p) 
            continue;
        dfs1(v, u);
        for(int i = 1; i <= m; ++i) {
            dp[u][i] += dp[v][i - 1];
        }
    }
}

void dfs2(int u, int p) {
    for(int i = 0; i < G[u].size(); ++i) {
        int v = G[u][i];
        if(v == p)
            continue;
        for(int i = m; i >= 2; --i) {
            dp[v][i] += dp[u][i - 1] - dp[v][i - 2];
        }
        dp[v][1]++;
        dfs2(v, u);
    }
}

int main()
{
    int u, v;
    while(~scanf("%d %d", &n, &m)) {
        for(int i = 1; i < n; ++i) {
            scanf("%d %d", &u, &v);
            G[u].push_back(v);
            G[v].push_back(u);
        }
        if(m == 1) {
            printf("%d
", n - 1);
        } else {
            dfs1(1, -1);
            dfs2(1, -1);
            LL ans = 0;
            for(int i = 1; i <= n; ++i) {
                ans += (LL)dp[i][m];
            }
            printf("%lld
", ans / 2);
        }
    }
    return 0;
}