From 71a735f9e0dc462c9e98faf3a383dad995ef1f9b Mon Sep 17 00:00:00 2001
From: Parundeep Singh <52928589+ParundeepSingh@users.noreply.github.com>
Date: Thu, 14 Oct 2021 16:36:06 +0530
Subject: [PATCH] Add Word Ladder (#2556)

---
 Strings/WordLadder.java | 97 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 97 insertions(+)
 create mode 100644 Strings/WordLadder.java

diff --git a/Strings/WordLadder.java b/Strings/WordLadder.java
new file mode 100644
index 000000000..4f461e76d
--- /dev/null
+++ b/Strings/WordLadder.java
@@ -0,0 +1,97 @@
+package Strings;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Arrays;
+import java.util.LinkedList;
+import java.util.Queue;
+import java.util.HashSet;
+
+/*
+    **Problem Statement:**
+    A transformation sequence from word beginWord to word endWord using a dictionary wordList is a sequence of words beginWord -> s1 -> s2 -> ... -> sk such that:
+
+    Every adjacent pair of words differs by a single letter.
+    Every si for 1 <= i <= k is in wordList. Note that beginWord does not need to be in wordList.
+    sk == endWord
+    Given two words, beginWord and endWord, and a dictionary wordList, return the number of words in the shortest transformation sequence from beginWord to endWord, or 0 if no such sequence exists.
+
+    **Example 1:**
+    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.
+
+    **Example 2:**
+    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.
+
+    **Constraints:**
+    1 <= beginWord.length <= 10
+    endWord.length == beginWord.length
+    1 <= wordList.length <= 5000
+    wordList[i].length == beginWord.length
+    beginWord, endWord, and wordList[i] consist of lowercase English letters.
+    beginWord != endWord
+    All the words in wordList are unique.
+*/
+
+class WordLadder {
+
+    /** Driver Code */
+    public static void main(String []args){
+
+        String beginWord = "hit";
+        String endWord = "cog";
+        String words[] = {"hot","dot","dog","lot","log","cog"}; 
+        List<String> wordList = Arrays.asList(words);
+
+        System.out.println("Ladder Length: " + ladderLength(beginWord,endWord,wordList));
+    }
+
+    /**
+     * This function finds the ladderLength
+     * @param beginWord: Starting word of the ladder 
+     * @param endWord: Ending word of the ladder
+     * @param wordList: This list contains the words which needs to be included in ladder.
+     * @return ladderLength: This function will return the ladderLength(level) if the endword is there. 
+     *  Otherwise, will return the length as 0.
+     */
+
+    public static int ladderLength(String beginWord, String endWord, List<String> wordList) {
+        HashSet<String> set = new HashSet();
+        for(String word : wordList){
+            set.add(word);
+        }
+        
+        if(!set.contains(endWord)) return 0;
+        
+        Queue<String> queue = new LinkedList();
+        queue.offer(beginWord);
+        int level = 1;
+        
+        while(!queue.isEmpty()){
+            int size = queue.size();
+            for(int i = 0; i < size; i++){
+                String curr = queue.poll();
+                char[] words_chars = curr.toCharArray();
+                for(int j = 0; j < words_chars.length; j++){
+                    char original_chars = words_chars[j];
+                    for(char c = 'a'; c <= 'z'; c++){
+                        if(words_chars[j] == c) continue;
+                        words_chars[j] = c;
+                        String new_word = String.valueOf(words_chars);
+                        if(new_word.equals(endWord)) return level+1;
+                        if(set.contains(new_word)){
+                            set.remove(new_word);
+                            queue.offer(new_word);
+                        }
+                    }
+                    words_chars[j] = original_chars;
+                }
+            }
+            level++;
+        }
+        return 0;
+    }
+}
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