Soyo

May Leetcode Challenge - Day 14

Implement Trie (Prefix Tree)

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

Example:

Trie trie = new Trie();

trie.insert("apple");
trie.search("apple");   // returns true
trie.search("app");     // returns false
trie.startsWith("app"); // returns true
trie.insert("app");   
trie.search("app");     // returns true

Note:

  • You may assume that all inputs are consist of lowercase letters a-z.
  • All inputs are guaranteed to be non-empty strings.

Solution

Each node in Trie keeps 3 information: value, list of its child-nodes, and is it a leaf node?
Should use hashtable to keep children.


class TrieNode<T: Hashable> {
    var value: T?
    
    weak var parent: TrieNode?
    var children = [T: TrieNode]()
    
    var isTerminate: Bool = false

    init(_ value: T? = nil, _ parent: TrieNode? = nil) {
        self.value = value
        self.parent = parent
    }

    func addChild(_ value: T) -> TrieNode? {
        guard children[value] == nil else {
            return nil
        }
        children[value] = TrieNode(value, self)
        return children[value]
    }
}

class Trie {
    typealias Node = TrieNode<Character>
    private var root: Node

    
    /** Initialize your data structure here. */
    init() {
        root = Node()
    }
    
    /** Inserts a word into the trie. */
    func insert(_ word: String) {
        guard !word.isEmpty else { return }
        
        var currentNode: Node? = root
        var count = 0
        for char in word.lowercased() {
            if let child = currentNode?.children[char] {
                currentNode = child
            }else {
                currentNode = currentNode?.addChild(char)
            }
            count += 1
            if count == word.count {
                currentNode?.isTerminate = true
            }
        }
    }
    
    /** Returns if the word is in the trie. */
    func search(_ word: String) -> Bool {
        guard !word.isEmpty else { return false }
        var currentNode: Node? = root
        
        for char in word.lowercased() {
            currentNode = currentNode?.children[char]
            if currentNode == nil { break }
        }
        
        return currentNode != nil && currentNode!.isTerminate
     }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    func startsWith(_ prefix: String) -> Bool {
        guard !prefix.isEmpty else { return false }
        var currentNode: Node? = root
        
        for char in prefix.lowercased() {
            currentNode = currentNode?.children[char]
            if currentNode == nil { break }
        }
        
        return currentNode != nil
    }
}

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