leetcode 87: Word Ladder II

Word Ladder II                                             Feb 11

Given two words (start and end), and a dictionary, find all shortest transformation sequence(s) fromstart to end, such that:

1. Only one letter can be changed at a time
2. Each intermediate word must exist in the dictionary

For example,

Given:
start = "hit"
end = "cog"
dict = ["hot","dot","dog","lot","log"]

Return

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

Note:

• All words have the same length.
• All words contain only lowercase alphabetic characters.

uncompleted.

public class Solution {
    public ArrayList<ArrayList<String>> findLadders(String start, String end,
   HashSet<String> dict) {
  // Start typing your Java solution below
  // DO NOT write main() function
  ArrayList<ArrayList<String>> res = new ArrayList<ArrayList<String>>();
  Queue<Node> queue = new LinkedList<Node>();
  queue.offer(new Node(start, null));
  HashSet<String> unique = new HashSet<String>();
        boolean first = false;
        int min = Integer.MIN_VALUE;
  while (!queue.isEmpty()) {
   Node n = queue.poll();
   String s = n.c;
   Sign: for (int i = 0; i < s.length(); i++) {
    for (char c = 'a'; c <= 'z'; c++) {
     StringBuilder sb = new StringBuilder(s);
     sb.setCharAt(i, c);
     String t = sb.toString();
     if (t.equals(end)) {
      ArrayList<String> match = new ArrayList<String>();
      match.add(end);
      while (n != null) {
       match.add(0, n.c);
       n = n.p;
      }
                        if(!first){
                first = true;
                            min = match.size();
      } else{
             if(match.size()>min) return res;   
      }
      res.add(match);
      
     }
     if (!unique.contains(t) && dict.contains(t)) {
      queue.offer(new Node(t, n));
                        unique.add(s);
     }
    }
   }
  }
  return res;
 }
 static class Node {
  public String c;
  public Node p;
  public Node(String c, Node p) {
   this.c = c;
   this.p = p;
  }
 }
}