Word Ladder.java

class Solution {
    public int ladderLength(String beginWord, String endWord, List<String> wordList) {
        HashSet<String> set=new HashSet<>(wordList);
        if(!set.contains(endWord)) return 0;
        Queue<String> q=new LinkedList<>();
        q.add(beginWord); q.add(null);
        HashSet<String> vis=new HashSet<>();
        vis.add(beginWord);
        int lvl=1;
        while(!q.isEmpty()) {
            String pop=q.poll();
            if(pop==null){
                lvl++;
                if(!q.isEmpty()) q.add(null);
            }else{
                char[] str=pop.toCharArray();
                for(int i=0;i<str.length; str[i]=pop.charAt(i), i++){
                    for(char a='a';a<='z';a++){
                        str[i]=a;
                        String neigh=new String(str);
                        if(set.contains(neigh) && !vis.contains(neigh)){
                            if(neigh.equals(endWord)) return lvl+1;
                            vis.add(neigh);
                            q.add(neigh);
                        }
                    }
                }
            }
        }
        return 0;
    }
}