Trie implementation compiles but crashes when entering data - c++

Question

I am using a trie implementation from topcoder.com and trying to store words in a trie structure. I used the code from the net. I can't seem to find out why my input is failing.
#include<iostream>
#include<queue>
#include<vector>
#include<algorithm>
#include<map>
#include<string>
using namespace std;
#define M 10000
#define tr(c,i) for(typeof(c.begin()) i=c.begin(); i!=c.end(); ++i)
class Trie {
public:
int words;
int prefixes;
Trie* edges[26];
Trie();
void addWord(string word);
int countWords(string word);
int countPrefixes(string prefix);
};
Trie::Trie() {
words = 0;
prefixes = 0;
for(int i=0;i<26;++i)
edges[i] = NULL;
}
void Trie::addWord(string word) {
if (word.size()==0)
words = words+1;
else {
prefixes = prefixes+1;
char k = word[0];
if (edges[k-'A']==NULL) {
edges[k-'A'] = new Trie;
}
edges[k-'A']->addWord(word.substr(1));
}
}
int Trie::countWords(string word) {
if (word.size()==0)
return words;
else {
char k = word[0];
if (edges[k-'A']==NULL)
return 0;
else {
return edges[k-'A']->countWords(word.substr(1));
}
}
}
int Trie::countPrefixes(string prefix) {
if (prefix.size()==0)
return prefixes;
else {
char k = prefix[0];
if (edges[k-'A']==NULL)
return 0;
else {
return edges[k-'A']->countPrefixes(prefix.substr(1));
}
}
}
int main()
{
int tests;
cin>>tests;
for(int t=1;t<=tests;t++)
{
Trie root;
int n;
cin>>n;
string str[n];
int ans=0;
for(int i=0;i<n;i++)
{
cin>>str[i];
root.addWord(str[i]);
}
for(int i=0;i<n;i++)
{
ans+=root.countPrefixes(str[i]);
}
cout<<ans;
}
return 0;
}
Any suggestions on what could be wrong? Thank you!