[Leetcode] Word Ladder II (Java)

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

Only one letter can be changed at a time
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.

找到所有最短变化路径

墨迹好久好久，怎么写怎么超时。。以后还得继续琢磨

public class Solution {    static class Node {        String          word;        ArrayList<Node> before;         Node(String word) {            this.word = word;            this.before = new ArrayList<Node>();        }    }     public ArrayList<ArrayList<String>> findLadders(String start, String end, HashSet<String> dict) {        ArrayList<ArrayList<String>> ret = new ArrayList<ArrayList<String>>();        char[] chs = new char[start.length()];         ArrayList<Node> now = new ArrayList<Node>(), next = null;        now.add(new Node(start));        dict.add(end);        dict.remove(start);        int length = start.length(), deep = 0;         while (now.size() > 0 && dict.contains(end)) {            deep++;             next = new ArrayList<Node>();            HashMap<String, Node> nextMap = new HashMap<String, Node>();            for (Node i : now) {                i.word.getChars(0, length, chs, 0);                for (int j = 0; j < length; j++) {                    char tmp = chs[j];                    for (char k = 'a'; k <= 'z'; k++) {                        chs[j] = k;                        String str = new String(chs);                         if (!dict.contains(str)) {                            continue;                        }                         Node node = nextMap.get(str);                        if (node == null) {                            node = new Node(str);                            nextMap.put(str, node);                            next.add(node);                        }                         node.before.add(i);                    }                    chs[j] = tmp;                }                           }            for (String key : nextMap.keySet()) {                dict.remove(key);            }            now = next;        }        if (dict.contains(end)) {            return ret;        }         Node root = null;        for (Node i : now) {            if (i.word.equals(end)) {                root = i;                break;            }        }         Node[] nodes = new Node[deep + 2];        nodes[deep] = new Node(start);         dfs(root, ret, nodes, 0, deep);         return ret;    }     private void dfs(Node root, ArrayList<ArrayList<String>> ret, Node[] nodes, int nodeIndex,                     int deep) {        if (nodeIndex == deep) {            ArrayList<String> tmp = new ArrayList<String>(deep);            for (int i = nodeIndex; i >= 0; i--) {                tmp.add(nodes[i].word);            }            ret.add(tmp);        } else {            nodes[nodeIndex] = root;            for (Node b : root.before) {                dfs(b, ret, nodes, nodeIndex + 1, deep);            }        }    }}

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