Word Ladder II

题目

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

1. Only one letter can be changed at a time
2. 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.

方法

算法思路和上一题基本上一致，仅仅须要使用一个数据结构。来保存每个结点的前驱结点。

    public List<List<String>> findLadders(String start, String end, Set<String> dict) {
    	Queue<String> queue = new LinkedList<String>();
    	Map<String, Integer> map = new HashMap<String, Integer>();
        Map<String, Set<String>> preSet = new HashMap<String, Set<String>>();
        queue.offer(start);
        map.put(start, 1);
        int minLadder = dict.size() + 2;
        boolean flag = false;
        while (!queue.isEmpty()) {
        	String str = queue.poll();
        	int count = map.get(str);
        	if  (count < minLadder) {
            	for (int i = 0; i < str.length(); i++) {
            		for (char k = 'a'; k <= 'z'; k++) {
            			if (str.charAt(i) != k) {
            				StringBuilder builder = new StringBuilder(str);
            				builder.setCharAt(i, k);
            				String temp = builder.toString();
            				if (temp.equals(end)) {
            					if (preSet.containsKey(temp)) {
            						preSet.get(temp).add(str);
            					} else {
            						Set<String> set = new HashSet<String>();
            						set.add(str);
            						preSet.put(temp, set);
            					}
            					minLadder = count + 1;
            					flag = true;
            				} else if (dict.contains(temp)) {
            					if (!map.containsKey(temp)) {
                					queue.offer(temp);
                					map.put(temp, count + 1);
            					} 
            					if (map.get(temp) > map.get(str)) {
                					if (preSet.containsKey(temp)) {
                						preSet.get(temp).add(str);
                					} else {
                						Set<String> set = new HashSet<String>();
                						set.add(str);
                						preSet.put(temp, set);
                					}
            					}

            				}
            				
            			}
            		}
            	}
        	} else {
        		break;
        	}
        }
        
        List<List<String>> list = new ArrayList<List<String>>();

        if (flag) {
            List<String> subList = new ArrayList<String>();
            subList.add(end);
        	getList(preSet, end, subList, list);
        	return list;
        } else {
        	return list;
        }
    }
    
    private void getList(Map<String, Set<String>> preSet, String cur,  List<String> subList, List<List<String>> list) {
    	if (preSet.containsKey(cur)) {
    		Set<String> set = preSet.get(cur);
    		for (String str : set) {
    			List<String> newList = new ArrayList<String>(subList);
    			newList.add(str);
    			getList(preSet, str, newList, list);
    		}
    	} else {
    		Collections.reverse(subList);
    		list.add(subList);
    	}
    }