1804

1804. Implement Trie II (Prefix Tree) (Medium)

A trie (pronounced as "try") or prefix tree is a tree data structure used to efficiently store and retrieve keys in a dataset of strings. There are various applications of this data structure, such as autocomplete and spellchecker.

Implement the Trie class:

• Trie() Initializes the trie object.
• void insert(String word) Inserts the string word into the trie.
• int countWordsEqualTo(String word) Returns the number of instances of the string word in the trie.
• int countWordsStartingWith(String prefix) Returns the number of strings in the trie that have the string prefix as a prefix.
• void erase(String word) Erases the string word from the trie.

 

Example 1:

Input
["Trie", "insert", "insert", "countWordsEqualTo", "countWordsStartingWith", "erase", "countWordsEqualTo", "countWordsStartingWith", "erase", "countWordsStartingWith"]
[[], ["apple"], ["apple"], ["apple"], ["app"], ["apple"], ["apple"], ["app"], ["apple"], ["app"]]
Output
[null, null, null, 2, 2, null, 1, 1, null, 0]

Explanation
Trie trie = new Trie();
trie.insert("apple");               // Inserts "apple".
trie.insert("apple");               // Inserts another "apple".
trie.countWordsEqualTo("apple");    // There are two instances of "apple" so return 2.
trie.countWordsStartingWith("app"); // "app" is a prefix of "apple" so return 2.
trie.erase("apple");                // Erases one "apple".
trie.countWordsEqualTo("apple");    // Now there is only one instance of "apple" so return 1.
trie.countWordsStartingWith("app"); // return 1
trie.erase("apple");                // Erases "apple". Now the trie is empty.
trie.countWordsStartingWith("app"); // return 0

 

Constraints:

• 1 <= word.length, prefix.length <= 2000
• word and prefix consist only of lowercase English letters.
• At most 3 * 104 calls in total will be made to insert, countWordsEqualTo, countWordsStartingWith, and erase.
• It is guaranteed that for any function call to erase, the string word will exist in the trie.

Companies: Docusign

Related Topics:
Hash Table, String, Design, Trie

Similar Questions:

• Implement Trie (Prefix Tree) (Medium)
• Encrypt and Decrypt Strings (Hard)

Hints:

• Try to solve the first version first and reuse your code.
• To implement the delete function, you should delete the trie nodes of the word if they are not shared with other words.
• You should keep for each trie node a counter of how many words share this node.

Solution 1.

If we don't really delete the node on erase.

// OJ: https://leetcode.com/problems/implement-trie-ii-prefix-tree/
// Author: github.com/lzl124631x
// Time:
//      Trie: O(1)
//      insert, countWordsEqualTo, countWordsStartingWith, erase: O(W)
// Space: O(1) extra space for all
struct TrieNode {
    TrieNode *next[26] = {};
    int cnt = 0, prefix = 0;
};
class Trie {
    TrieNode root;
    TrieNode *find(string s) {
        auto n = &root;
        for (char c : s) {
            if (!n->next[c - 'a'] || n->next[c - 'a']->prefix == 0) return nullptr;
            n = n->next[c - 'a'];
        }
        return n;
    }
public:
    void insert(string s) {
        auto n = &root;
        for (char c : s) {
            if (!n->next[c - 'a']) n->next[c - 'a'] = new TrieNode();
            n = n->next[c - 'a'];
            n->prefix++;
        }
        n->cnt++;
    }
    int countWordsEqualTo(string s) {
        auto n = find(s);
        return n ? n->cnt : 0;
    }
    int countWordsStartingWith(string prefix) {
        auto n = find(prefix);
        return n ? n->prefix : 0;
    }
    void erase(string s) {
        TrieNode *n = &root;
        for (char c : s) {
            n = n->next[c - 'a'];
            --n->prefix;
        }
        --n->cnt;
    }
};

If we really need to delete the node on erase.

// OJ: https://leetcode.com/problems/implement-trie-ii-prefix-tree
// Author: github.com/lzl124631x
// Time:
//      Trie: O(1)
//      insert, countWordsEqualTo, countWordsStartingWith, erase: O(W)
// Space: O(1) extra space for all
struct TrieNode {
    TrieNode *next[26] = {};
    int cnt = 0, prefix = 0;
};
class Trie {
    TrieNode root;
    TrieNode *find(string s) {
        auto n = &root;
        for (char c : s) {
            if (!n->next[c - 'a']) return nullptr;
            n = n->next[c - 'a'];
        }
        return n;
    }
public:
    void insert(string s) {
        auto n = &root;
        for (char c : s) {
            if (!n->next[c - 'a']) n->next[c - 'a'] = new TrieNode();
            n = n->next[c - 'a'];
            n->prefix++;
        }
        n->cnt++;
    }
    int countWordsEqualTo(string s) {
        auto n = find(s);
        return n ? n->cnt : 0;
    }
    int countWordsStartingWith(string prefix) {
        auto n = find(prefix);
        return n ? n->prefix : 0;
    }
    void erase(string s) {
        TrieNode *n = &root, *next = n->next[s[0] - 'a'];
        for (int N = s.size(), i = 0; i < N; ++i) {
            auto prev = n;
            n = next;
            if (i + 1 < N) next = n->next[s[i + 1] - 'a'];
            if (--n->prefix == 0) {
                if (prev) prev->next[s[i] - 'a'] = nullptr;
                delete n;
                n = nullptr;
            }
        }
        if (n) --n->cnt;
    }
};