208. Implement Trie (Prefix Tree) --M

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

Example:

Trie trie = new Trie();

trie.insert("apple");
trie.search("apple");   // returns true
trie.search("app");     // returns false
trie.startsWith("app"); // returns true
trie.insert("app");   
trie.search("app");     // returns true

Note:

• You may assume that all inputs are consist of lowercase letters a-z.
• All inputs are guaranteed to be non-empty strings.

A:

不要搞得太复杂，就是每个character一个节点就行了了。（外加HashMap）
（ empty String “”  是所有 字符串的前缀。~~~即使还没有开始存）

class Trie {
public:
    /** Initialize your data structure here. */
    Trie() {
        root = new Node();
    }
    
    /** Inserts a word into the trie. */
    void insert(string word) {
        Node *cur = root;
        for(int i =0;i<word.length();++i)
        {
            char ch = word[i];
            if(cur->chmap.find(ch) == cur->chmap.end())
            {
                Node* tmp = new Node();
                cur->chmap[ch] = tmp;
            }             
            cur = cur->chmap.at(ch);
        }
        cur->isLeaf = true;
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        Node *cur = root;        
        for(int i =0;i<word.length();++i)
        {
            char ch = word[i];
            if(cur->chmap.find(ch)==cur->chmap.end())
            {
                return false;
            }
            cur = cur->chmap.find(ch)->second;
        }
        return cur->isLeaf;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
                Node *cur = root;        
        for(int i =0;i<prefix.length();++i)
        {
            char ch = prefix[i];
            if(cur->chmap.find(ch)==cur->chmap.end())
            {
                return false;
            }
            cur = cur->chmap.find(ch)->second;
        }
        return true;
    }
private:
    struct Node{
        unordered_map<char,Node*> chmap;
        bool isLeaf=false;
    };
    Node * root; // root contains no char to reach it
};

/**
 * Your Trie object will be instantiated and called as such:
 * Trie* obj = new Trie();
 * obj->insert(word);
 * bool param_2 = obj->search(word);
 * bool param_3 = obj->startsWith(prefix);
 */

Mistakes:
1：  一开始加上了，如果该节点后面没有，就自己保存一个string，搞得太复杂
2： 这道题，最容易错的地方是，search和 startWith中，最后要检查是否是null节点。