Create 2025-04-29-Leetcode-127.md

Author: hyeonukim

_posts/Leetcode/Graphs/2025-04-29-Leetcode-127.md

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+---
+title: Leetcode 127. Word Ladder
+description: Explanation for Leetcode 127 - Word Ladder, and its solution in Python.
+date: 2025-04-29
+categories: [Leetcode, Graphs, Hard]
+tags: [Leetcode, Python, Study, Graphs, Hard]
+math: true
+---
+
+## Problem
+[Leetcode 127 - Word Ladder](https://leetcode.com/problems/word-ladder/description/)
+
+Example:
+```
+Input: beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log","cog"]
+Output: 5
+Explanation: One shortest transformation sequence is "hit" -> "hot" -> "dot" -> "dog" -> cog", which is 5 words long.
+
+Input: beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log"]
+Output: 0
+Explanation: The endWord "cog" is not in wordList, therefore there is no valid transformation sequence.
+```
+
+## Approach
+
+We can use BFS to find the words that have similar words in wordList. Once we find the correct word, we can return the counts that we've searched through
+
+Here is the Python code for the solution:
+```python
+class Solution:
+    def ladderLength(self, beginWord: str, endWord: str, wordList: List[str]) -> int:
+        if beginWord == endWord or endWord not in wordList:
+            return 0
+        
+        words, res = set(wordList), 0
+        q = deque([beginWord])
+        while q:
+            res += 1
+            for _ in range(len(q)):
+                node = q.popleft()
+                if node == endWord:
+                    return res
+                for i in range(len(node)):
+                    for c in range(97, 123):
+                        if chr(c) == node[i]:
+                            continue
+                        nei = node[:i] + chr(c) + node[i+1:]
+                        if nei in words:
+                            q.append(nei)
+                            words.remove(nei)
+
+        return 0    
+```
+## Time Complexity and Space Complexity
+
+Time Complexity: $O(m^2 * n)$
+
+Space Complexity: $O(m^2 * n)$