【LeetCode 208】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.

思路：

构建一个简单的字典树，要求实现插入、查找、查找前缀三个操作。这里使用map实现对子树的构建（一方面方便查找，另一方面是懒 - -！），但是效率并不是太高，LeetCode上提交后跑了147ms，不是很理想，不过不影响对字典树这种树形结构的理解。

 1 class TrieNode {
 2 public:
 3     // Initialize your data structure here.
 4     TrieNode(char c):ch(c), isEnd(false){}
 5
 6     map<char, TrieNode *> childNode;
 7
 8     char ch;
 9     bool isEnd;
10 };
11
12 class Trie {
13 public:
14     Trie() {
15         root = new TrieNode(‘#‘);
16     }
17
18     // Inserts a word into the trie.
19     void insert(string s) {
20
21         TrieNode *t = root;
22
23         //从root遍历待插入字符串，若c存在则继续向下遍历，反之则新建一个分支
24         for(int i = 0; i < s.length(); i++)
25         {
26             char c = s[i];
27
28             map<char, TrieNode*>::iterator it = t->childNode.find(c);
29             if(it == t->childNode.end())
30             {
31                 TrieNode *newNode = new TrieNode(c);
32                 t->childNode.insert(make_pair(c, newNode));
33                 t = newNode;
34             }
35             else
36             {
37                 t = t->childNode[c];
38             }
39         }
40         t->isEnd = true;
41     }
42
43     // Returns if the word is in the trie.
44     bool search(string key) {
45         TrieNode *s = root;
46
47         //从root遍历待查找字符串，若c存在则向下遍历，反之则不存在
48         for(int i = 0; i < key.length(); i++)
49         {
50             char c = key[i];
51
52             map<char, TrieNode*>::iterator it = s->childNode.find(c);
53             if(it == s->childNode.end())
54                 return false;
55             else
56                 s = s->childNode[c];
57         }
58         //返回最后一个结点的状态，判断是否为一个word
59         return s->isEnd;
60     }
61
62     // Returns if there is any word in the trie
63     // that starts with the given prefix.
64     bool startsWith(string prefix) {
65         TrieNode *s = root;
66
67         //与search一致
68         for(int i = 0; i < prefix.length(); i++)
69         {
70             char c = prefix[i];
71
72             map<char, TrieNode*>::iterator it = s->childNode.find(c);
73
74             if(it == s->childNode.end())
75                 return false;
76             else
77                 s = s->childNode[c];
78         }
79         //只要找到即返回true
80         return true;
81     }
82
83 private:
84     TrieNode* root;
85 };