[LeetCode] Word Ladder

Thought:
It is a BFS problem.

Code:
import java.util.*;
public class Solution {
    public int ladderLength(String start, String end, HashSet<String> dict) {
        LinkedList<String> q1 = new LinkedList<String>(); // as a queue
        LinkedList<Integer> q2 = new LinkedList<Integer>(); // as a queue
        q1.offer(start);
        q2.offer(1);
        dict.remove(start);
        while (!q1.isEmpty()) {
            String current = q1.poll();
            int depth = q2.poll();
            if (current.equals(end)) return depth;
            Iterator<String> it = dict.iterator();
            while(it.hasNext()) {
                String tmp = it.next();
                if (adjacent(tmp, current)) {
                    q1.offer(tmp);
                    q2.offer(depth + 1);    
                    it.remove();
                }                
            }
        }
        return 0;
    }
    public boolean adjacent(String a, String b) {
        int result = 0;
        for (int i = 0; i < a.length(); i++) {
            if (a.charAt(i) != b.charAt(i)) result++;
        }
        return result == 1;
    }
}