LeetCode Tries Prefix Tree

 1 class TrieNode {
 2 public:
 3     const static int NR_FANOUT = 26;
 4     TrieNode* child[NR_FANOUT];
 5     int count;
 6     // Initialize your data structure here.
 7     TrieNode() {
 8         for (int i=0; i<NR_FANOUT; i++) {
 9             child[i] = NULL;
10         }
11         count = 0;
12     }
13 };
14
15 class Trie {
16 public:
17     Trie() {
18         root = new TrieNode();
19     }
20
21     // Inserts a word into the trie.
22     void insert(string s) {
23         insert(s, 0, root);
24     }
25
26     // Returns if the word is in the trie.
27     bool search(string key) {
28         return search(key, 0, root) != NULL;
29     }
30
31     // Returns if there is any word in the trie
32     // that starts with the given prefix.
33     bool startsWith(string prefix) {
34         return search_prefix(prefix, 0, root) != NULL;
35     }
36 private:
37     TrieNode* insert(string& s, int pos, TrieNode* current) {
38         int len = s.size();
39         if (pos > len) {
40             return NULL;
41         }
42         if (current == NULL) {
43             current = new TrieNode();
44         }
45         if (pos == len) {
46             current->count++;
47             return current;
48         }
49         int idx = s[pos] - ‘a‘;
50         current->child[idx] = insert(s, pos + 1, current->child[idx]);
51         return current;
52     }
53
54     TrieNode* search(string& s, int pos, TrieNode* current) {
55         if (current == NULL) {
56             return NULL;
57         }
58         int len = s.size();
59         if (pos > len) {
60             return NULL;
61         } else if (pos == len && current->count) {
62             return current;
63         }
64         return search(s, pos + 1, current->child[s[pos] - ‘a‘]);
65     }
66     TrieNode* search_prefix(string& s, int pos, TrieNode* current) {
67         if (current == NULL) {
68             return NULL;
69         }
70         int len = s.size();
71         if (pos >= len) {
72             return current;
73         }
74         return search_prefix(s, pos + 1, current->child[s[pos] - ‘a‘]);
75     }
76 private:
77     TrieNode* root;
78 };

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

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