[LeetCode-JAVA] Word Ladder II

public class Solution {
    private class Node{
        public Node pre;
        public String val;
        public Node(Node p, String s){
            pre = p;
            val = s;
        }
    }
    public List<List<String>> findLadders(String start, String end, Set<String> dict) {
        dict.add(start);
        dict.add(end); //注意（1）
        Map<String, Set<String>> neighbours = calNeighbours(dict);
        
        List<List<String>> req = new ArrayList<List<String>>();
        
        LinkedList<Node> queue = new LinkedList<Node>();
        queue.offer(new Node(null, start)); //开始节点的前一个为null
        
        Set<String> visited = new HashSet<String>(); //注意（4）
        boolean flag = false; //注意（3）
        while(!queue.isEmpty() || visited.size() == dict.size()){
            int num = queue.size();
            for(int i = 0 ; i < num ; i++){
                Node n = queue.poll();
                if(end.equals(n.val)){
                    findPath(n, req);
                    flag = true; //注意（3）
                }else{
                    Set<String> temp = neighbours.get(n.val);
                    if(temp == null || temp.size() == 0)
                        continue;
                    else{
                        for(String s : temp){
                            if(!visited.contains(s)){ //注意（4）
                                queue.offer(new Node(n, s));
                            }
                        }
                    }
                }
                visited.add(n.val);//注意（4）
            }
            if(flag)
                break;
        }
        if(!flag) // 如果flag为false，证明无解
            return new ArrayList<List<String>>();
        return req;
    }
    
    private void findPath(Node n, List<List<String>> req) {
        // TODO Auto-generated method stub
        List<String> temp = new ArrayList<String>();
        while(n != null){
            temp.add(0, n.val);
            n = n.pre;
        }
        req.add(temp);
    }
    public Map<String, Set<String>> calNeighbours(Set<String> dict) {
        Map<String, Set<String>> neighbours = new HashMap<String, Set<String>>();
        
        for(String str : dict){
            int len = str.length();
            char[] chars = str.toCharArray();
            
            for(int i = 0 ; i < len ; i++){
                char old = chars[i]; //注意（2）
                for(char c = ‘a‘ ; c <= ‘z‘ ; c++){
                    if(c == chars[i])
                        continue;
                    chars[i] = c;
                    String newstr = new String(chars);
                    
                    if(dict.contains(newstr)) {
                        Set<String> set = neighbours.get(str);
                        if(set != null)
                            set.add(newstr);
                        else{
                            Set<String> newset = new HashSet<String>();
                            newset.add(newstr);
                            neighbours.put(str, newset);
                        }
                    }
                    chars[i] = old ; //注意（2）引用型变量 每一次用完要还原 
                } // end fot c
            } // end for len
        } // end for str

        return neighbours;
    }
}