package Strings;

import java.util.List;
import java.util.Arrays;
import java.util.LinkedList;
import java.util.Queue;
import java.util.HashSet;

/*
    **Problem Statement:**
    A transformation sequence from word beginWord to word endWord using a dictionary wordList is a sequence of words beginWord -> s1 -> s2 -> ... -> sk such that:

    Every adjacent pair of words differs by a single letter.
    Every si for 1 <= i <= k is in wordList. Note that beginWord does not need to be in wordList.
    sk == endWord
    Given two words, beginWord and endWord, and a dictionary wordList, return the number of words in the shortest transformation sequence from beginWord to endWord, or 0 if no such sequence exists.

    **Example 1:**
    Input: beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log","cog"]
    Output: 5 
    Explanation: One shortest transformation sequence is "hit" -> "hot" -> "dot" -> "dog" -> cog", which is 5 words long.

    **Example 2:**
    Input: beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log"]
    Output: 0
    Explanation: The endWord "cog" is not in wordList, therefore there is no valid transformation sequence.

    **Constraints:**
    1 <= beginWord.length <= 10
    endWord.length == beginWord.length
    1 <= wordList.length <= 5000
    wordList[i].length == beginWord.length
    beginWord, endWord, and wordList[i] consist of lowercase English letters.
    beginWord != endWord
    All the words in wordList are unique.
*/

class WordLadder {

    /** Driver Code */
    public static void main(String []args){

        String beginWord = "hit";
        String endWord = "cog";
        String words[] = {"hot","dot","dog","lot","log","cog"}; 
        List<String> wordList = Arrays.asList(words);

        System.out.println("Ladder Length: " + ladderLength(beginWord,endWord,wordList));
    }

    /**
     * This function finds the ladderLength
     * @param beginWord: Starting word of the ladder 
     * @param endWord: Ending word of the ladder
     * @param wordList: This list contains the words which needs to be included in ladder.
     * @return ladderLength: This function will return the ladderLength(level) if the endword is there. 
     *  Otherwise, will return the length as 0.
     */

    public static int ladderLength(String beginWord, String endWord, List<String> wordList) {
        HashSet<String> set = new HashSet();
        for(String word : wordList){
            set.add(word);
        }
        
        if(!set.contains(endWord)) return 0;
        
        Queue<String> queue = new LinkedList();
        queue.offer(beginWord);
        int level = 1;
        
        while(!queue.isEmpty()){
            int size = queue.size();
            for(int i = 0; i < size; i++){
                String curr = queue.poll();
                char[] words_chars = curr.toCharArray();
                for(int j = 0; j < words_chars.length; j++){
                    char original_chars = words_chars[j];
                    for(char c = 'a'; c <= 'z'; c++){
                        if(words_chars[j] == c) continue;
                        words_chars[j] = c;
                        String new_word = String.valueOf(words_chars);
                        if(new_word.equals(endWord)) return level+1;
                        if(set.contains(new_word)){
                            set.remove(new_word);
                            queue.offer(new_word);
                        }
                    }
                    words_chars[j] = original_chars;
                }
            }
            level++;
        }
        return 0;
    }
}