Question

I have a specific question regarding the Trie data structure and what is going wrong with my code. Function parameter root always is NULL when I recursively call insert. Here is my code:

Code:

//subNodes is an array of TrieNode pointers that contains indices for all letters in the alphabet

bool insert(const string& word, TrieNode* root, int curI = 0)
//PRE:  word must be a valid word in a dictionary
//POST: True when a word is inserted into the Trie, false otherwise
{
    if(curI >= word.length())        //word has been scanned fully
    {
        root->isWord = true;
        return true;
    }
    else                             //word has more letters to be scanned
    {
        if(root->subNodes[word[curI] - 'A'] == NULL)    //if the current letter of the word is not in the trie
        {                                            //   insert the letter and advance the current letter of the word
            root->subNodes[word[curI] - 'A'] = new TrieNode(word[curI]);
            insert(word, root->subNodes[word[curI] - 'A'], curI++);
        }
        else                                         //if the currrent letter of the word is in the trie
        {                                            //   advance the current letter of the word
            insert(word, root->subNodes[word[curI] - 'A'], curI++);
        }
    }

}

I tested this by replacing subNodes[word[curI] - 'A'] with subNodes[word[13]] (13 is the index for N in the alphabet and I was testing the word not) and root was no longer NULL for that call. Therefore something is going wrong with the indexing. Does anyone know what is wrong? I considered using a C++ map or vector. Does anybody have any disagreements with using an array?

Answer 1

Did you mean ++curl – i.e. pass in the incremented value to the recursion call? As it stands you’re passing the same value into each recursion since curl++ is post-increment. It may be easier to just write curl + 1 anyway since you don’t need the curl value again.