2025-01-07 v. 7.7.6: added "126. Word Ladder II"

README.md

@@ -645,3 +645,4 @@ Profile on LeetCode: [fartem](https://leetcode.com/fartem/).
 | 32. Longest Valid Parentheses                 | [Link](https://leetcode.com/problems/longest-valid-parentheses/)                 | [Link](./lib/hard/32_longest_valid_parentheses.rb)                 | [Link](./test/hard/test_32_longest_valid_parentheses.rb)                 |
 | 41. First Missing Positive                    | [Link](https://leetcode.com/problems/first-missing-positive/)                    | [Link](./lib/hard/41_first_missing_positive.rb)                    | [Link](./test/hard/test_41_first_missing_positive.rb)                    |
 | 115. Distinct Subsequences                    | [Link](https://leetcode.com/problems/distinct-subsequences/)                     | [Link](./lib/hard/115_distinct_subsequences.rb)                    | [Link](./test/hard/test_115_distinct_subsequences.rb)                    |
+| 126. Word Ladder II                           | [Link](https://leetcode.com/problems/word-ladder-ii/)                            | [Link](./lib/hard/126_word_ladder_ii.rb)                           | [Link](./test/hard/test_126_word_ladder_ii.rb)                           |

leetcode-ruby.gemspec

@@ -5,7 +5,7 @@ require 'English'
 ::Gem::Specification.new do |s|
   s.required_ruby_version = '>= 3.0'
   s.name = 'leetcode-ruby'
-  s.version = '7.7.5'
+  s.version = '7.7.6'
   s.license = 'MIT'
   s.files = ::Dir['lib/**/*.rb'] + %w[README.md]
   s.executable = 'leetcode-ruby'

lib/hard/126_word_ladder_ii.rb

@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+# https://leetcode.com/problems/word-ladder-ii/
+# @param {String} begin_word
+# @param {String} end_word
+# @param {String[]} word_list
+# @return {String[][]}
+def find_ladders(begin_word, end_word, word_list)
+  return [] unless (unvisited = word_list.to_set).include?(end_word)
+
+  transformations = ::Hash.new { |h, k| h[k] = ::Set.new }
+
+  begin_layer = ::Set[begin_word]
+  end_layer = ::Set[end_word]
+  unvisited.delete(end_word)
+  transforms = Array('a'..'z').product(Array(0...begin_word.size))
+
+  current_levels = begin_layer
+  previous_levels = end_layer
+  loop do
+    current_levels, previous_levels = previous_levels, current_levels
+
+    unvisited.subtract(previous_levels)
+
+    next_layer = ::Set.new
+    current_levels.each do |word|
+      transforms.each do |c, i|
+        next if word[i] == c
+
+        transform = word.dup.tap { |w| w[i] = c }
+
+        is_neighbor = unvisited.include?(transform)
+
+        next_layer << transform if is_neighbor
+
+        if is_neighbor || previous_levels.include?(transform)
+          if current_levels.equal?(begin_layer)
+            transformations[word] << transform
+          else
+            transformations[transform] << word
+          end
+        end
+      end
+    end
+
+    break if next_layer.empty? || begin_layer.any? { |w| transformations[w].intersect?(end_layer) }
+
+    current_levels.replace(next_layer)
+  end
+
+  dfs_paths(begin_word, end_word, transformations)
+end
+
+private
+
+# @param {String} begin_word
+# @param {String} end_word
+# @param {Hash} word_graph
+# @return {String[][]}
+def dfs_paths(begin_word, end_word, word_graph)
+  paths = []
+  path = []
+  stack = [[begin_word, 1]]
+
+  until stack.empty?
+    word, level = stack.pop
+
+    path.pop until path.size < level
+    path << word
+
+    if word == end_word
+      paths << path.dup
+    else
+      word_graph[word].each { |w| stack << [w, level + 1] }
+    end
+  end
+
+  paths
+end

test/hard/test_126_word_ladder_ii.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require_relative '../test_helper'
+require_relative '../../lib/hard/126_word_ladder_ii'
+require 'minitest/autorun'
+
+class WordLadderIITest < ::Minitest::Test
+  def test_default_one
+    assert_equal(
+      [
+        %w[hit hot lot log cog],
+        %w[hit hot dot dog cog]
+      ],
+      find_ladders(
+        'hit',
+        'cog',
+        %w[hot dot dog lot log cog]
+      )
+    )
+  end
+
+  def test_default_two
+    assert_equal(
+      [],
+      find_ladders(
+        'hit',
+        'cog',
+        %w[hot dot dog lot log]
+      )
+    )
+  end
+end