Word-Ladder

def ladderLength(beginWord, endWord, wordList):
    
    word_set = set(wordList)
    
    if endWord not in word_set:
        return 0
    
    begin_set = {beginWord}
    end_set = {endWord}
    
    visited = set()
    level = 1
    
    while begin_set and end_set:
        
        # Always expand smaller set (optimization)
        if len(begin_set) > len(end_set):
            begin_set, end_set = end_set, begin_set
        
        next_set = set()
        
        for word in begin_set:
            for i in range(len(word)):
                for ch in 'abcdefghijklmnopqrstuvwxyz':
                    
                    new_word = word[:i] + ch + word[i+1:]
                    
                    if new_word in end_set:
                        return level + 1
                    
                    if new_word in word_set and new_word not in visited:
                        visited.add(new_word)
                        next_set.add(new_word)
        
        begin_set = next_set
        level += 1
    
    return 0


# User Input
beginWord = input("Enter begin word: ")
endWord = input("Enter end word: ")
wordList = input("Enter word list (space separated): ").split()

print("Shortest transformation length:", ladderLength(beginWord, endWord, wordList))