void print(TrieNode* root, string t,vector<string>& s ){
int cnt_nulls = 0;
for(int i = 0; i < 26; i++){
if(root->v[i] != NULL){
char c = (i + 'a');
// cout << c << endl;
t.push_back(c);
print(root->v[i] , t, s);
t.pop_back();
}
else cnt_nulls++;
}
if(cnt_nulls == 26)
s.push_back(t);
}