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Leetcode NO.208 Implement Trie (Prefix Tree)

2015-06-24 06:02 393 查看
本题题目要求如下:

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

本题的难点在于理解Trie这个数据结构,之前我也不了解,做完这道题后,稍微有了一点认识,Trie相当一种字典树,使用Trie的方法跟我们使用字典的方法很像,比如查词“Rocklee'”,先找第一个字母R,找到了之后,R有26个childnode,查看指向o的node是否为空,如果为空,则找不到,如果不为空,则持续的找下去。。。而查字典也是找到R,然后找R开始里面第二个字母是O的。。然后持续的找下去。。

如果要详细理解,并完成题目的话,要看一看下面两个连接,
https://www.topcoder.com/community/data-science/data-science-tutorials/using-tries/ http://www.geeksforgeeks.org/trie-insert-and-search/
。。了解了数据结构后,这题就不难了,并没有太多的技巧和算法,纯是基本功问题,代码如下:

class TrieNode {
public:
    // Initialize your data structure here.
    TrieNode* children[26];
    bool leaf;
    TrieNode() {
        leaf = false;
        fill_n(children, 26, nullptr);
    }
};

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

    // Inserts a word into the trie.
    void insert(string word) {
        TrieNode* node = root;
        int index = 0;
        for (int i = 0; i < word.length(); ++i) {
            index = word[i] - 'a';
            if (node->children[index] == nullptr) {
                node->children[index] = new TrieNode();
            }
            node = node->children[index];
        }
        node->leaf = true;
    }

    // Returns if the word is in the trie.
    bool search(string word) {
        TrieNode* node = root;
        int index = 0;
        for (int i = 0; i < word.size(); ++i) {
            index = word[i] - 'a';
            if (node->children[index] == nullptr) {
                return false;
            }
            node = node->children[index];
        }
        return (node != nullptr and node->leaf == true);
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    bool startsWith(string prefix) {
        TrieNode* node = root;
        int index = 0;
        for (int i = 0; i < prefix.size(); ++i) {
            index = prefix[i] - 'a';
            if (node->children[index] == nullptr) {
                return false;
            }
            node = node->children[index];
        }
        return (node != nullptr);
    }

private:
    TrieNode* root;
};

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