From 5dd403e69ab53c363799e0182e6443e2fde99d7a Mon Sep 17 00:00:00 2001
From: StriveDD <Kip2021czb@gmail.com>
Date: Thu, 2 Mar 2023 11:21:03 +0800
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---
 problems/0127.单词接龙.md | 67 ++++++++++++++++++++++++++++++++++-
 1 file changed, 66 insertions(+), 1 deletion(-)

diff --git a/problems/0127.单词接龙.md b/problems/0127.单词接龙.md
index 7ffd6a21..b4078913 100644
--- a/problems/0127.单词接龙.md
+++ b/problems/0127.单词接龙.md
@@ -16,7 +16,7 @@
 * 转换过程中的中间单词必须是字典 wordList 中的单词。
 * 给你两个单词 beginWord 和 endWord 和一个字典 wordList ，找到从 beginWord 到 endWord 的 最短转换序列 中的 单词数目 。如果不存在这样的转换序列，返回 0。
 
- 
+
 示例 1：
 
 * 输入：beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log","cog"]
@@ -134,7 +134,71 @@ public int ladderLength(String beginWord, String endWord, List<String> wordList)
 }
 ```
 
+## Java 双向BFS
+
+```java
+class Solution {
+    // 判断单词之间是否之差了一个字母
+    public boolean isValid(String currentWord, String chooseWord) {
+        int count = 0;
+        for (int i = 0; i < currentWord.length(); i++)
+            if (currentWord.charAt(i) != chooseWord.charAt(i)) ++count;
+        return count == 1;
+    }
+
+    public int ladderLength(String beginWord, String endWord, List<String> wordList) {
+        if (!wordList.contains(endWord)) return 0;      // 如果 endWord 不在 wordList 中，那么无法成功转换，返回 0
+
+        // ansLeft 记录从 beginWord 开始 BFS 时能组成的单词数目
+        // ansRight 记录从 endWord 开始 BFS 时能组成的单词数目
+        int ansLeft = 0, ansRight = 0;
+        
+        // queueLeft 表示从 beginWord 开始 BFS 时使用的队列
+        // queueRight 表示从 endWord 开始 BFS 时使用的队列
+        Queue<String> queueLeft = new ArrayDeque<>(), queueRight = new ArrayDeque<>();
+        queueLeft.add(beginWord);
+        queueRight.add(endWord);
+
+        // 从 beginWord 开始 BFS 时把遍历到的节点存入 hashSetLeft 中
+        // 从 endWord 开始 BFS 时把遍历到的节点存入 hashSetRight 中
+        Set<String> hashSetLeft = new HashSet<>(), hashSetRight = new HashSet<>();
+        hashSetLeft.add(beginWord);
+        hashSetRight.add(endWord);
+
+        // 只要有一个队列为空，说明 beginWord 无法转换到 endWord
+        while (!queueLeft.isEmpty() && !queueRight.isEmpty()) {
+            ++ansLeft;
+            int size = queueLeft.size();
+            for (int i = 0; i < size; i++) {
+                String currentWord = queueLeft.poll();
+                // 只要 hashSetRight 中存在 currentWord，说明从 currentWord 可以转换到 endWord
+                if (hashSetRight.contains(currentWord)) return ansRight + ansLeft;
+                for (String chooseWord : wordList) {
+                    if (hashSetLeft.contains(chooseWord) || !isValid(currentWord, chooseWord)) continue;
+                    hashSetLeft.add(chooseWord);
+                    queueLeft.add(chooseWord);
+                }
+            }
+            ++ansRight;
+            size = queueRight.size();
+            for (int i = 0; i < size; i++) {
+                String currentWord = queueRight.poll();
+                // 只要 hashSetLeft 中存在 currentWord，说明从 currentWord 可以转换到 beginWord
+                if (hashSetLeft.contains(currentWord)) return ansLeft + ansRight;
+                for (String chooseWord : wordList) {
+                    if (hashSetRight.contains(chooseWord) || !isValid(currentWord, chooseWord)) continue;
+                    hashSetRight.add(chooseWord);
+                    queueRight.add(chooseWord);
+                }
+            }
+        }
+        return 0;
+    }
+}
+```
+
 ## Python 
+
 ```
 class Solution:
     def ladderLength(self, beginWord: str, endWord: str, wordList: List[str]) -> int:
@@ -301,3 +365,4 @@ function diffonechar(word1: string, word2: string): boolean {
 <a href="https://programmercarl.com/other/kstar.html" target="_blank">
   <img src="../pics/网站星球宣传海报.jpg" width="1000"/>
 </a>
+