PathsBetweenWords

package com.yusufaytas.test.intercom;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

public class PathsBetweenWords {

	public List<List<String>> getPaths(String beginWord, String endWord, Set<String> words) {
		List<List<String>> allPaths = new ArrayList<List<String>>();
		Map<String, Set<String>> wordMap = createWordMap(words);
		getPaths(allPaths, new ArrayList<String>(), beginWord, endWord, wordMap);
		return allPaths;
	}

	private void getPaths(List<List<String>> allPaths, List<String> path, String beginWord, String endWord,
			Map<String, Set<String>> wordMap) {
		List<String> currentPath = new ArrayList<String>(path);
		currentPath.add(beginWord);
		if(beginWord.equals(endWord)){
			allPaths.add(currentPath);
		}
		else{
			Set<String> nextWords = wordMap.get(beginWord);
			for (String nextWord : nextWords) {
				if(!currentPath.contains(nextWord)){
					//avoid the loops.
					getPaths(allPaths, currentPath, nextWord, endWord, wordMap);
				}
			}
		}
	}
	
	private Map<String, Set<String>> createWordMap(Set<String> words){
		Map<String, Set<String>> wordMap = new HashMap<String, Set<String>>(words.size());
		for (String word : words) {
			Set<String> nextWords = new HashSet<String>();
			for (String nextWord : words) {
				if(isItNextWord(word, nextWord)){
					nextWords.add(nextWord);
				}
			}
			wordMap.put(word, nextWords);
 		}
		return wordMap;
	}
	
	private boolean isItNextWord(String word, String nextWord){
		boolean firstChange = false;
		for (int i = 0; i < word.length(); i++) {
			if(word.charAt(i) != nextWord.charAt(i)){
				if(firstChange){
					return false;
				}
				firstChange = true;
			}
		}
		return firstChange;
	}
}