127. Word ladder

 1     public int ladderLength(String beginWord, String endWord, Set<String> wordList) {
 2         if(beginWord == null || endWord == null || endWord.length() != beginWord.length() || wordList == null || wordList.size() == 0) {
 3             return 0;
 4         }
 5         int len = beginWord.length();
 6         Set<String> beginSet = new HashSet<String>();
 7         Set<String> endSet = new HashSet<String>();
 8         Set<String> visited = new HashSet<String>();
 9         beginSet.add(beginWord);
10         endSet.add(endWord);
11         visited.add(beginWord);
12         int dist = 1;
13         while(!beginSet.isEmpty() && !endSet.isEmpty()) {
14             if(beginSet.size() > endSet.size()) {
15                 Set<String> temp = beginSet;
16                 beginSet = endSet;
17                 endSet = temp;
18             }
19             Set<String> toAdd = new HashSet<String>();
20             for(String each: beginSet) {
21                 char[] chs = each.toCharArray();
22                 for(int i = 0; i < len; i++) {
23                     char ch = chs[i];
24                     for(char c = ‘a‘; c <= ‘z‘; c++) {
25                         chs[i] = c;
26                         String tempWord = new String(chs);
27                         if(endSet.contains(tempWord)) {
28                             return ++dist;
29                         }
30                         if(wordList.contains(tempWord) && !visited.contains(tempWord)) {
31                             toAdd.add(tempWord);
32                             wordList.remove(tempWord);
33                             visited.add(tempWord);
34                         }
35                     }
36                     chs[i] = ch;
37                 }
38             }
39             dist++;
40             beginSet = toAdd;
41         }
42         return 0;
43     }