I'm trying to solve the problem "Islands" of the following contest

http://codeforces.com/gym/101291/attachments/download/5256/20162017-acmicpc-pacific-northwest-regional-contest-div-2-en.pdf

This is my simple BFS solution, which produces a runtime error on testcase 21. Does anybody have an explanation?

#include <iostream>
#include <queue>

using namespace std;

void check_position(int i, int k, char** grid, queue<pair<int,int> >& q){
  if(grid[i][k]=='L' || grid[i][k]=='C'){
    grid[i][k] = 'X';
    q.push(pair<int,int>(i,k));
  }
}

void bfs(int i, int k, char** grid, int n, int m){
  queue<pair<int,int> > q;
  grid[i][k]='X';
  q.push(pair<int,int>(i,k));
  pair<int,int> temp;
  while(!q.empty()){
    temp = q.front();
    q.pop();
    if (temp.first>0) check_position(temp.first-1,temp.second, grid, q);
    if (temp.second>0) check_position(temp.first,temp.second-1, grid, q);
    if (temp.second<m-1) check_position(temp.first,temp.second+1, grid, q);
    if (temp.first<n-1) check_position(temp.first+1,temp.second, grid, q);
  }
}

int main(){
  int n,m;
  cin>>n>>m;
  char** grid = new char* [n];
  for (int i=0;i<n;i++) grid[i] = new char[m];
  for (int i=0;i<n;i++) cin>>grid[i];
  int island_count = 0;
  for (int i=0;i<n;i++){
    for (int k=0;k<m;k++){
      if (grid[i][k]=='L'){
        island_count++;
        bfs(i,k,grid,n,m);
      }
    }
  }
  cout<<island_count<<"\n";
}