Distance_in_Tree.md

Algorithm: Graph Tree

Problem: Distance in Tree

#include <bits/stdc++.h>
using namespace std;
//#include<ext/pb_ds/assoc_container.hpp>
//#include<ext/pb_ds/tree_policy.hpp>
//using namespace __gnu_pbds;
//typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> pbds; // find_by_order, order_of_key
#define ll long long
#define vi vector<int>
#define pii pair<int, int>
#define pll pair<ll, ll>
#define vvi vector<vi>
#define vll vector<ll>
#define vvll vector<vector<ll>>
#define vpii vector<pair<int,int>>
#define vpll vector<pair<ll,ll>>
int Set(int N, int pos) {return  N = N | (1<<pos);}
int Reset(int N, int pos) {return  N = N & ~(1<<pos);}
bool Cheek(int N, int pos) {return  (bool)(N & (1<<pos));}
#define least_one_pos(x) __builtin_ffs(x)
#define leading_zeros(x) __builtin_clz(x)
#define tailing_zeros(x) __builtin_ctz(x)
#define num_of_one(x) __builtin_popcount(x)
///............x...........///
#define all(a) a.begin(), a.end()
#define allr(a) a.rbegin(), a.rend()
#define mp(a, b) make_pair(a, b)
#define pb push_back
#define UNIQUE(X) (X).erase(unique(all(X)), (X).end())
#define ft cout << "for test"<<endl;
#define print(v) for (auto it : v)cout << it<<" ";cout << endl;
#define PI acos(-1.0)
#define FIO ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define t_c int test, cs = 1;cin>>test;while (test--)
///................function.....................///

//#define mod 1000000007
//........constant........///
const ll N = 5e4+5;
void file(){
   freopen("input.txt","r",stdin);
   freopen("output.txt","w",stdout);
}
ll mem[N][501],n,k,ans=0;
vvi g(N);
void dfs(int nd, int par){
	mem[nd][0]=1;
	for(auto x:g[nd]){
		if(par==x)continue;
		dfs(x,nd);
		for(int i=1; i<=k; i++)
			ans += mem[x][i-1]*mem[nd][k-i];
		for(int i=1; i<=k; i++)
			mem[nd][i]+=mem[x][i-1];
	}
}
int main(){
	FIO;
	// file();
	cin>>n>>k;
	for(int i=1; i<n; i++){
		int a,b;
		cin>>a>>b;
		g[a].push_back(b);
		g[b].push_back(a);
	}
	dfs(1,1);
	cout<<ans<<endl;
}