Segmentation fault in c++ code unable to find reason - c++

Question

I am getting segmentation for the last test case (Don't know what it is ) while solving the problem GREATESC.
Concept of the problem is basic bfs. Given an undirected graph |V| <= 3500 and |E| <= 1000000
Find the minimum distance between two given vertices.
Here's the problem link http://opc.iarcs.org.in/index.php/problems/GREATESC
Here's my solution link
http://ideone.com/GqTc6k
#include <iostream>
#include <stdio.h>
#include <cmath>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <cassert>
#include <ctime>
#include <cstdlib>
#include <algorithm>
#define Pi 3.14159
#define vi vector<int>
#define pi pair<int,int>
#define si stack<int>
typedef long long int ll;
using namespace std;
bool b[3501][3501]={0};
int main ()
{
int n,m;
cin >>n>>m;
int u,v;
for (int i =1;i<= m;i++)
{
scanf("%d",&u);
scanf("%d",&v);
b[u][v]=b[v][u]=1;
}
// input completed.
int dist[n+1];
int h,V;
cin >>h>>V;
dist[h]=0;
//cout<<"hero "<<h<<" "<<V<<endl;
queue<int> q;
bool bfs[3501];
for (int i=1;i<= n;i++)bfs[i]=1;
q.push(h);
bfs[h]=0;
while (!q.empty())
{
int top = q.front();
// cout<<top<<endl;
q.pop();
for (int i = 1 ;i <= 3500;i++)
{
if(bfs[i] && b[i][top])
{
int x = i;
dist[i] = dist[top] +1;
if(x == V){cout<<dist[x]<<endl;return 0;}
bfs[x]=0;
q.push(x);
}
}
}
cout<<0<<endl;
}

Answer 1

You have this:
cin >>n>>m;
...
int dist[n+1];
Hence the array dist may have size less than 3500. But:
for (int i = 1 ;i <= 3500;i++)
...
dist[i] = dist[top] +1;
This code might be indexing outside of dist.
You seem to need in general to be more careful that when indexing into an array, you're inside the bounds of the array.
Consider using std::vector instead of arrays, then indexing with at to get bounds checking. Or alternatively, manually assert that values are within range:
#include <assert.h>
...
for (int i = 1 ;i <= 3500;i++)
...
assert(i >= 0 && i <= n && top >= 0 && top <= n);
dist[i] = dist[top] +1;