LeetCode Implement Trie (Prefix Tree)

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

Note:

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

思路分析：这题主要考察Trie 即前缀树的实现，Trie可以用于字典的压缩存储，可以节省空间，但是不节省时间（和HashSet相比）。这题实质是实现一颗多叉树（分支因子26）的插入和查找操作。定义每个TrieNode保存char c，保存一个HashMap用于储存所有的孩子节点，key是对应的字符，value是孩子节点，定义flag hasWord来标记这个node上是否存在word。对于插入操作，直接从上向下分层扫描树，如果没有对应字符节点存在就新建节点，如果有就去相应路径向下探察。对于查询操作，前缀树的定义可以保证当我们从前向后扫描字符串时候，每一个字符都可以从上到下对应前缀树的每一层，所以扫描过程中如果有任何一个字符不存在于当前层中，就可以立刻停止查找返回null，也就是不存在这样的word或者前缀，否则继续从对应的分支向下探察。时间复杂度是O（L），L是树的高度，也就是最长word的长度，空间复杂度26^L，每一层分支因子26，有L层。

AC Code

class TrieNode {    // Initialize your data structure here.    //0908    char c;    HashMap<Character, TrieNode> children = new HashMap<Character, TrieNode>();    boolean hasWord;        public TrieNode(){            }        public TrieNode(char c){        this.c = c;    }}public class Trie {    private TrieNode root;    public Trie() {        root = new TrieNode();    }    // Inserts a word into the trie.    public void insert(String word) {        TrieNode cur = root;        HashMap<Character, TrieNode> curChildren = root.children;        char[] wordArray = word.toCharArray();        for(int i = 0; i < wordArray.length; i++){            char wc = wordArray[i];            if(curChildren.containsKey(wc)){                cur = curChildren.get(wc);            } else {                TrieNode newNode = new TrieNode(wc);                curChildren.put(wc, newNode);                cur = newNode;            }            curChildren = cur.children;            if(i == wordArray.length - 1){                cur.hasWord= true;            }        }    }    // Returns if the word is in the trie.    public boolean search(String word) {        if(searchWordNodePos(word) == null){            return false;        } else if(searchWordNodePos(word).hasWord)           return true;          else return false;    }    // Returns if there is any word in the trie    // that starts with the given prefix.    public boolean startsWith(String prefix) {        if(searchWordNodePos(prefix) == null){            return false;        } else return true;    }        public TrieNode searchWordNodePos(String s){        HashMap<Character, TrieNode> children = root.children;        TrieNode cur = null;        char[] sArray = s.toCharArray();        for(int i = 0; i < sArray.length; i++){            char c = sArray[i];            if(children.containsKey(c)){                cur = children.get(c);                children = cur.children;            } else{                return null;            }        }        return cur;    }}// Your Trie object will be instantiated and called as such:// Trie trie = new Trie();// trie.insert("somestring");// trie.search("key");//0918

关于trie的好的博文 http://dongxicheng.org/structure/trietree/

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