wenbao与树上dp

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http://codeforces.com/problemset/problem/161/D

求树上路径长度为k的个数

#include "iostream"
#include <vector>
using namespace std;

const int maxn = 50009;
vector<int> v[maxn];
int n, k, cnt = 0, d[maxn][505];

//d[i][j] 经过点i长度为j的路径个数

void dd(int x, int pre){
    d[x][0] = 1;
    for(int i = 0; i < v[x].size(); ++i){
        int xx = v[x][i];
        if(xx != pre){
            dd(xx, x);
            for(int j = 0; j < k; ++j) cnt += d[x][j]*d[xx][k-j-1];
            for(int j = 1; j <= k; ++j) d[x][j] += d[xx][j-1];
        }
    }
}

int main(){
#ifdef wenbao
    freopen("in", "r", stdin);
#endif
    int x, y;
    scanf("%d%d", &n, &k);
    for(int i = 1; i < n; ++i){
        scanf("%d%d", &x, &y);
        v[x].push_back(y), v[y].push_back(x);
    }
    dd(1, 0);
    printf("%d
", cnt);
    return 0;
}

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