bad alloc exception when trying to resolve BFS challenge on HackerRank - c++

Question

So i was trying to make the challage: Breadth First Search: Shortest Reach on HackerRank, but i keep getting the bad alloc exception when the tests have great numbers of node/edges. The program works on the first test, so i don't think, it's something wrong with the implementation.
So here is the implementation:
(sorry for the indentation , my first question)
#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <queue>
#include <limits.h>
using namespace std;
int main() {
//test numbers
int t;
//the cost
int cost = 6;
cin >> t;
//for each test
for (int nt = 0; nt < t; ++nt) {
int n, e;
int snode;
queue <int> *que = new queue<int>();
//read the node/edges
cin >> n >> e;
//distance/visited/parents arrays/adjlist vector
int dist[n + 1] = {-1};
bool visited[n + 1] = {false};
int parents[n + 1] = {-1};
vector< vector<int> > adjList(n + 1);
//read into the adjlist, unoriented graph, each edge has 6 weight
for (int ne = 0; ne < e; ++ne) {
int x, y;
cin >> x >> y;
adjList[x].push_back(y);
adjList[y].push_back(x);
}
//read the starting node
cin >> snode;
dist[snode] = 0;
//do actual bfs
que->push(snode);
visited[snode] = true;
while(!que->empty()) {
int c_node = que->front();
que->pop();
for (int i = 0; i < adjList[c_node].size(); ++i) {
if (visited[adjList[c_node].at(i)] == false) {
que->push(adjList[c_node].at(i));
parents[adjList[c_node].at(i)] = c_node;
dist[adjList[c_node].at(i)] = dist[parents[adjList[c_node].at(i)]] + cost;
visited[adjList[c_node].at(i)] == true;
}
}
}
//print at output the distance from the starting node to each other node
//if unreachable, print -1
for (int i = 1; i < n + 1; ++i) {
if (i == snode) {
} else if (dist[i] == 0 && i != snode) {
cout << "-1 ";
} else {
cout << dist[i] << " ";
}
}
cout << "\n";
}
return 0;
}
Am i doing something wrong, i haven't seen anyone else complain on this matter in the discussion section of the site.
How can i avoid the exception to be thrown and from where does it come?
Thank you!

Answer 1

I don't know, exactly, what is the cause of your exception; and I don't know ho to reproduce your problem because depends (I suppose) from the input values. A lot of input values, I suppose.
But I see some weak points (IMHO) of your code, so I try to point your attention to them.
1) you alloc a std::queue in your for cycle
queue <int> *que = new queue<int>();
but you never free it; it's a waste of memory
2) you're using C-style variable-length arrays
int dist[n + 1] = {-1};
bool visited[n + 1] = {false};
int parents[n + 1] = {-1};
They aren't valid C++ standard code. I suggest you the use of standard containers (std::vector or std::queue).
3) you're initializing your C-style variable-length arrays with a initializers lists with only an element (-1 or false). I suppose your intention was initialize all n+1 elements with -1 and false. But this syntax initialize only the first element of the array with -1 and false.
If you want to initialize all n+1 element to -1 and false, the solution is (again) use standard containers; by example
std::vector<int> dist(n+1, -1);
std::vector<bool> visited(n+1, false);
std::vector<int> parents(n+1, -1);
4) you access arrays without bounds checking. By example:
cin >> snode;
dist[snode] = 0;
where snode is a int variable; if you insert a negative value, or a value over n, you write dist out of its bounds, devastating the memory. This, I suppose, can explain your "bad alloc exception".
Suggestion: use standard containers (again) instead of C-style array and use at() (that perform bounds checking) instead []; so
cin >> snode;
dist.at(snode) = 0;
5) sorry for my bad English (ok, I'm joking: this isn't one of your weak points; this is one of mine).