Word Ladder II(BFS + DFS)

Given two words (beginWord and endWord), and a dictionary's word list, find all shortest transformation sequence(s) from beginWord to endWord, such that:

Only one letter can be changed at a time
Each intermediate word must exist in the word list

For example,

Given:

beginWord =

"hit"

endWord =

"cog"

wordList =

["hot","dot","dog","lot","log"]

Return

[
    ["hit","hot","dot","dog","cog"],
    ["hit","hot","lot","log","cog"]
  ]

import java.util.*;

public class Solution {
	public List<List<String>> findLadders(String start, String end, Set<String> dict) {
		List<List<String>> ladders = new ArrayList<List<String>>();
		Map<String, List<String>> map = new HashMap<String, List<String>>();
		Map<String, Integer> distance = new HashMap<String, Integer>();

		dict.add(start);
		dict.add(end);

		bfs(map, distance, start, end, dict);

		List<String> path = new ArrayList<String>();

		dfs(ladders, path, end, start, distance, map);

		return ladders;
	}
	void dfs(List<List<String>> ladders, List<String> path, String crt, String start, Map<String, Integer> distance,
			Map<String, List<String>> map) {
		path.add(crt);
		if (crt.equals(start)) {
			Collections.reverse(path);
			ladders.add(new ArrayList<String>(path));
			Collections.reverse(path);
		} else {
			for (String next : map.get(crt)) {
				if (distance.containsKey(next) && distance.get(crt) == distance.get(next) + 1) {
					dfs(ladders, path, next, start, distance, map);
				}
			}
		}
		path.remove(path.size() - 1);
	}
	void bfs(Map<String, List<String>> map, Map<String, Integer> distance, String start, String end, Set<String> dict) {
		Queue<String> q = new LinkedList<String>();
		q.offer(start);
		distance.put(start, 0);
		for (String s : dict) {
			map.put(s, new ArrayList<String>());
		}
		while (!q.isEmpty()) {
			String crt = q.poll();
			List<String> nextList = expand(crt, dict);
			for (String next : nextList) {
				map.get(next).add(crt);
				if (!distance.containsKey(next)) {
					distance.put(next, distance.get(crt) + 1);
					q.offer(next);
				}
			}
		}
	}
	//在dict中找，看crt替换单个字符后有没有在dict中的，如果有，放在expansion中
	List<String> expand(String crt, Set<String> dict) {
		List<String> expansion = new ArrayList<String>();

		for (int i = 0; i < crt.length(); i++) {
			for (char ch = 'a'; ch <= 'z'; ch++) {
				if (ch != crt.charAt(i)) {
					String expanded = crt.substring(0, i) + ch + crt.substring(i + 1);
					if (dict.contains(expanded)) {
						expansion.add(expanded);
					}
				}
			}
		}
		return expansion;
	}
	public static void main(String[] args) {
		Solution solution = new Solution();
		String start = "hit"; 
		String end = "cog"; 
		Set<String> dict = new HashSet<>();
		dict.add("hot");
		dict.add("dot");
		dict.add("dog");
		dict.add("lot");
		dict.add("log");
		List res = solution.findLadders("hit", "cog", dict);
		System.out.println(res);
	}
}