How to implement the startsWith function of a trie in python

Question

I have the following implementation of a trie in python.

I want to implement the startsWith function that will return a list of all words that start with the prefix.

Currently it return tue or false. Im not sure how to get a list of words in my implementation. I believe I need some sort of dfs, but not sure how to implement it

class TrieNode:
    def __init__(self):
        self.children = {}
        self.end_of_word = False

class Trie(object):
    def __init__(self):
        self.root = TrieNode()

    def insert(self, word):
        current = self.root
        for char in word:
            if char not in cur.children:
                current.children[char] = TrieNode()
            current = current.children[char]
        current.end_of_word = True
            
    
    def search(self, word):
        current = self.root
        for char in word:
            if char not in current.children:
                return False
            current = current.children[char]
        return current.end_of_word
        
    def startsWith(self, prefix):
        current = self.root
        for char in prefix:
            if char not in current.children:
                return False
            current = current.children[char]
        return True
    
    def remove(self, word):
        current = self.root
        if self.search(word):
            for char in word:
                current = current.children[char]
            current.end_of_word = False
        


# Your Trie object will be instantiated and called as such:
obj = Trie()
obj.insert('apple')
obj.insert('applegate')
obj.insert('apply')
obj.insert('application')

print(obj.search('applegate'))
obj.remove('applegate')
print(obj.search('applegate'))

print(obj.startsWith('app')) -> should return a list of all words that start with 'app'

You received answers below. Could you leave some feedback and mark an answer as accepted if appropriate? — trincot, Feb 08 '22 at 08:17

score 2 · Accepted Answer · answered Feb 07 '22 at 18:31

As you have logic in search and startsWith that is common, I would put that common logic in a separate method (I'll call it locate). Then on the TreeNode class you could define a recursive method that finds (yields) all words rooted at that node.

Code:

class TrieNode:
    def __init__(self):
        self.children = {}
        self.end_of_word = False

    def allWords(self):
        if self.end_of_word:
            yield ""
        for char, child in self.children.items():
            for word in child.allWords():
                yield char + word

class Trie(object):
    def __init__(self):
        self.root = TrieNode()

    def insert(self, word):
        current = self.root
        for char in word:
            if char not in current.children:
                current.children[char] = TrieNode()
            current = current.children[char]
        current.end_of_word = True
            
    def locate(self, word):
        current = self.root
        for char in word:
            if char not in current.children:
                return
            current = current.children[char]
        return current

    def search(self, word):
        current = self.locate(word)
        return current and current.end_of_word
        
    def startsWith(self, prefix):
        current = self.locate(prefix)
        if not current:
            return []
        return [prefix + word for word in current.allWords()]
    
    def remove(self, word):
        current = self.root
        if self.search(word):
            for char in word:
                current = current.children[char]
            current.end_of_word = False

This works like a charm. thanks – user18065369 Feb 08 '22 at 19:26 — user18065369, Feb 08 '22 at 19:26

score 0 · Answer 2 · answered Feb 07 '22 at 18:20

You need something like:

def all_children_iterator(self, prefix: str) -> Iterator[str]:
    # returns all children of the current node with prefix prepended
    if self.end_of_word:
         yield prefix
    for char, child in self.children.items():
         yield from child.all_children_iterator(prefix + char)

Then instead of returning True, you return list(current.all_children_iterator(''))

This code could be made slightly more efficient by using a list to hold the letters and judiciously appending and popping them. But that makes the basic idea a little bit harder to understand.

How to implement the startsWith function of a trie in python

2 Answers2