problem 126

p126.java

@@ -0,0 +1,90 @@
+/*
+126. Word Ladder II
+
+Given two words (beginWord and endWord), and a dictionary's word list, find all shortest transformation sequence(s) from beginWord to endWord, such that:
+
+Only one letter can be changed at a time
+Each transformed word must exist in the word list. Note that beginWord is not a transformed word.
+For example,
+
+Given:
+beginWord = "hit"
+endWord = "cog"
+wordList = ["hot","dot","dog","lot","log","cog"]
+
+Return
+  [
+    ["hit","hot","dot","dog","cog"],
+    ["hit","hot","lot","log","cog"]
+  ]
+*/
+
+public class Solution {
+
+    public List<List<String>> findLadders(String beginWord, String endWord, List<String> wordList) {
+        Map<String, Integer> map = new HashMap<>();
+        Map<String, Set<String>> neighbors = new HashMap<>();
+        neighbors.put(beginWord, new HashSet<>());
+        map.put(beginWord, -1);
+        Set<String> wordSet = new HashSet<>();
+        for(String word : wordList){
+            wordSet.add(word);
+            map.put(word, -1);
+            neighbors.put(word, new HashSet<>());
+        }
+        int distance = 0;
+        boolean found = false;
+        Queue<String> queue = new LinkedList<>();
+        queue.offer(beginWord);
+        while(!queue.isEmpty()){
+            int len = queue.size();
+            for(int i = 0; i < len; i++){
+                String curWord = queue.poll();
+                if(map.get(curWord) == -1 || map.get(curWord) > distance){
+                    map.put(curWord, distance);
+                }
+                if(curWord.equals(endWord)){
+                    found = true;
+                }
+                for(int j = 0; j < curWord.length(); j++){
+                    char[] arr = curWord.toCharArray();
+                    for(char c = 'a'; c <= 'z'; c++){
+                        if(c != arr[j]){
+                            arr[j] = c; 
+                        }
+                        String newWord = new String(arr);
+                        if(wordSet.contains(newWord)){
+                            if(map.get(newWord) == -1){
+                                queue.offer(newWord);
+                                neighbors.get(curWord).add(newWord);
+                            }
+                        }
+                    }
+                }
+            }
+            if(found) break;
+            distance++;
+        }
+        List<List<String>> wraplist = new ArrayList<>();
+        dfs(map, neighbors, new ArrayList<>(), wraplist, beginWord, endWord);
+        return wraplist;
+    }
+
+    public void dfs(Map<String, Integer> map, Map<String, Set<String>> neighbors, List<String> sublist, List<List<String>> wraplist, String curWord, String endWord){
+
+        if(curWord.equals(endWord)){
+            sublist.add(curWord);
+            wraplist.add(new ArrayList<>(sublist));
+            sublist.remove(sublist.size() - 1);
+            return;
+        }
+
+        for(String neighbor : neighbors.get(curWord)){
+            sublist.add(curWord);
+            if(map.get(neighbor) == map.get(curWord) + 1){
+                dfs(map, neighbors, sublist, wraplist, neighbor, endWord);
+            }
+            sublist.remove(sublist.size() - 1);
+        }      
+    }
+}