【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 };