Implement Trie (Prefix Tree)

Implement a trie with insert, search, and startsWith methods.

Note:
You may assume that all inputs are consist of lowercase letters a-z.

Analyse: For each node, it has 26 children, from a-z. 

1. For the string insertion operation, the procedure is:

    --Set root as the temp node;

    --For every character in the string, compute its corresponding place at the branch, and mark them.

    --After all characters are marked, set isVal to indicate it's a value rather than a prefix.

2. For the search operation, the procedure is:

    --Set root as the temp node;

　 --For every character in the string, continuously check the existance of it. If NULL node encountered, jump out the of loop.

    --If the node is empty, then return false; Else, we need to judge whether this string is a value or a prefix.

3. For the startWith operation, it's quite similiar to the search operation except that all search operations with true result returned can return true in the start operation. 

Runtime: 64ms.

class TrieNode {
public:
    // Initialize your data structure here.
    TrieNode* childNode[26];
    bool isVal;//to indicate whether the leaf node is the target word or the prefix
    TrieNode() {
        isVal = false;
        for(int i = 0; i < 26; i++)
            childNode[i] = NULL;
    }
};

class Trie {
public:
    Trie() {
        root = new TrieNode();
    }

    // Inserts a word into the trie.
    void insert(string word) {
        TrieNode* temp = root;
        for(int i = 0; i < word.size(); i++){
            if(temp->childNode[word[i] - 'a'] == NULL)
                temp->childNode[word[i] - 'a'] = new TrieNode();
            temp = temp->childNode[word[i] - 'a'];
        }
        temp->isVal = true; //if the word is inserted, then isVal is true
    }

    // Returns if the word is in the trie.
    bool search(string word) {
        TrieNode* temp = root;
        for(int i = 0; i < word.size(); i++){
            if(temp == NULL) break;
            temp = temp->childNode[word[i] - 'a'];
        }
        if(!temp) return false; //if the route does not exist, return false
        return temp->isVal;  
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    bool startsWith(string prefix) {
        TrieNode* temp = root;
        for(int i = 0; i < prefix.size(); i++){
            if(temp == NULL) break;
            temp = temp->childNode[prefix[i] - 'a'];
        }
        if(temp) 
            return true; //if the route exists, return true
        else 
            return false; //if the route doesn't exists, return false
    }

private:
    TrieNode* root;
};

// Your Trie object will be instantiated and called as such:
// Trie trie;
// trie.insert("somestring");
// trie.search("key");