Level_wise_node_count.cpp

// Level wise node count
#include <bits/stdc++.h>
using namespace std;
#define INF 1<<30
#define MAX 100005
#define FASTIO ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
typedef long long ll;
vector<int>graph[MAX];
map<int,int>mp;

int level[MAX];
bool visit[MAX];

int bfs(int s, int x)
{
    queue<int>Q;
    Q.push(s);
    level[s] = 0;
    visit[s] = true;
    mp[level[s]]++;

    while(!Q.empty())
    {
        int u = Q.front();
        Q.pop();

        for(int i = 0; i < graph[u].size(); i++){
            int v = graph[u][i];
            if(visit[v] == false){
                level[v] = level[u]+1;
                mp[level[v]]++;
                Q.push(v);
                visit[v] = true;
            }
        }
    }
    return mp[x-1];
}

int main()
{
    FASTIO
    ///*
    //double start_time = clock();
#ifndef ONLINE_JUDGE
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
    freopen("error.txt", "w", stderr);
#endif
    //*/

    int n;
    int a,b;
    cin >> n;
    for(int i = 0; i < n-1; i++){
        cin >> a >> b;
        graph[a].push_back(b);
        graph[b].push_back(a);
    }
    int x;
    cin >> x;
    cerr << a << endl;
    cout << bfs(1, x)<<endl;
    //for(int i = 1; i <= 20; i++){
    //    cerr << mp[i] << endl;
    //}
    //double end_time = clock();
    //printf( "Time = %lf ms\n", ( (end_time - start_time) / CLOCKS_PER_SEC)*1000);

    return 0;
}