Broken Judge? Gives different answer for same test case

#1

This is a solution that I came up with

struct node {
    map<char, node*> next;
};
node * root = new node();
void insert(string s, node* root) {
    node * temp = root;
    for(int i=0;i<s.size();i++) {
        if(temp->next.find(s[i]) != temp->next.end()) {
            // char exists
            temp = temp->next[s[i]];
        } else {
            temp->next[s[i]] = new node();
            temp = temp->next[s[i]];
        }
    }
}
string findLongestCommonPrefix(node* root) {
    node * temp = root;
    string res = "";
    while(temp->next.size()==1) {
        res += temp->next.begin()->first;
        temp = temp->next.begin()->second;  
    }
    return res;
}
string Solution::longestCommonPrefix(vector<string> &A) {
    for(int i=0;i<A.size();i++) {
        insert(A[i], root);
    }
    return findLongestCommonPrefix(root);
}

and for the input

A : [ "abcd", "abde", "abcf" ]

my code returns
ab

when I test it as custom input. But the same code returns an empty string when using the Test button.
Can someone help me?