UVa : 336 :: A Node Too Far

Problem : Please find the problem here.

Explanation : The problem basically asked and suggest to traverse the graph with BFS from a given node upto a given level. Remember that, BFS visit vertices that are direct neighbors of the source vertex (first layer), neighbors of direct neighbors (second layer), and so on, layer by layer. To identify which layer the given vertex belongs to, we can push the vertex with its level from the starting node in the queue. One more thing, the number of vertices are not given, so I made a edge-list first, mapped all the elements to the integers starting from zero, later created an adjacency-list with the given edge-list.

Code : Used BFS.

#include <bits/stdc++.h>
//#define debug
using namespace std;

const int mxN = 1e3;
int n, m;
vector<int> adj[mxN];
map<int, int> node_id;
vector<array<int, 2>> edges;
bool vis[mxN];

int bfs(int node, int time){

    memset(vis, false, sizeof(vis));

    queue<array<int, 2>> qu;
    qu.push({node, time});
    vis[node] = 1;
    while(qu.size()){
	array<int, 2> u = qu.front();
	qu.pop();
	if(u[1]){
	    for(int v : adj[u[0]]){
		if(!vis[v]){
		    qu.push({v, u[1]-1});
		    vis[v] = 1;
		}
	    }
	}

    }
    // now counting the NOT_VISITED nodes;
    int cnt = 0;
    for(int i = 0; i < n; i++){
        if(!vis[i]){
	    cnt++;
	}
    }
    return cnt;
}

void init(){

    edges.clear();

    node_id.clear();

    memset(vis, false, sizeof(vis));
    for(int i = 0; i < n; i++){
	adj[i].clear();
    }
}

void set_adj(){
    for(auto it : edges){
	adj[it[0]].emplace_back(it[1]);
	adj[it[1]].emplace_back(it[0]);
    }
}

int main()
{
    int __case = 1;
    while(cin >> m){
	int id = 0;
	init();
	for(int i = 0, u, v; i < m; i++){
	    cin >> u >> v;
	    if(node_id.find(u) == node_id.end()){
#ifdef debug
	        cout << u << " is mapped to " << id << '\n';
#endif
	        node_id[u] = id++;
	    }
	    if(node_id.find(v) == node_id.end()){
#ifdef debug

                cout << v << " v is mapped to " << id << '\n';
#endif
		node_id[v] = id++;
	    }
	    u = node_id[u], v = node_id[v];
	    edges.push_back({u, v});
	}
	n = node_id.size();
	set_adj();
#ifdef debug
	cout << "printing adjacency list\n";
	for(int i = 0; i < n; i++){
	    cout << i << "->";
	    for(int u : adj[i]){
		cout << u << ' ';
	    }
	    cout << '\n';
	}
#endif
	int node, time;
	while(cin >> node >> time && (node || time)){
	    printf("Case %d: %d nodes not reachable from node %d with TTL = %d.\n",__case++, bfs(node_id[node], time), node, time);
	}
    }	
    return 0;
}