'Word Ladder II' soln

Author: joewnga

leetcode/word_ladder_ii.rs

@@ -0,0 +1,135 @@
+/// 
+/// Problem: Word Ladder II
+/// 
+/// 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 all the shortest 
+/// transformation sequences from beginWord to endWord, or an empty list if no such sequence exists.
+/// Each sequence should be returned as a list of the words [beginWord, s1, s2, ..., sk].
+/// 
+/// Example 1:
+/// Input: beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log","cog"]
+/// Output: [["hit","hot","dot","dog","cog"],["hit","hot","lot","log","cog"]]
+/// Explanation: There are 2 shortest transformation sequences:
+/// "hit" -> "hot" -> "dot" -> "dog" -> "cog"
+/// "hit" -> "hot" -> "lot" -> "log" -> "cog"
+/// 
+/// Example 2:
+/// Input: beginWord = "hit", endWord = "cog", wordList = ["hot","dot","dog","lot","log"]
+/// Output: []
+/// Explanation: The endWord "cog" is not in wordList, so there is no valid transformation sequence.
+/// 
+/// Constraints:
+/// 1 <= beginWord.length <= 5
+/// endWord.length == beginWord.length
+/// 1 <= wordList.length <= 500
+/// wordList[i].length == beginWord.length
+/// beginWord, endWord, and wordList[i] consist of lowercase English letters.
+/// beginWord != endWord
+/// All the words in wordList are unique.
+/// 
+
+// # Solution
+// Time complexity: O(N * M^2) where N is the length of wordList and M is the length of each word
+// Space complexity: O(N^2) 
+
+use std::collections::{HashSet, HashMap, VecDeque};
+impl Solution {
+    pub fn find_ladders(begin_word: String, end_word: String, word_list: Vec<String>) -> Vec<Vec<String>> {
+        let mut word_set: HashSet<String> = word_list.into_iter().collect();
+       
+       
+       if !word_set.contains(&end_word) {
+           return vec![];
+       }
+       
+       word_set.remove(&begin_word);
+       
+       let mut queue = VecDeque::new();
+       queue.push_back(begin_word.clone());
+       
+       let mut parents: HashMap<String, Vec<String>> = HashMap::new();
+       
+       let mut current_level = HashSet::new();
+       current_level.insert(begin_word.clone());
+       
+       let mut found = false;
+       
+       while !queue.is_empty() && !found {
+           let level_size = queue.len();
+           
+           let mut level_visited = HashSet::new();
+           
+           for _ in 0..level_size {
+               let current_word = queue.pop_front().unwrap();
+               
+               let word_chars: Vec<char> = current_word.chars().collect();
+               
+               for i in 0..word_chars.len() {
+                   let mut new_chars = word_chars.clone();
+                   
+                   for c in b'a'..=b'z' {
+                       new_chars[i] = c as char;
+                       let new_word: String = new_chars.iter().collect();
+                       
+                       if !word_set.contains(&new_word) {
+                           continue;
+                       }
+                       
+                       parents.entry(new_word.clone())
+                              .or_insert_with(Vec::new)
+                              .push(current_word.clone());
+                       
+                       if !level_visited.contains(&new_word) {
+                           level_visited.insert(new_word.clone());
+                           
+                           if new_word == end_word {
+                               found = true;
+                           } else {
+                               queue.push_back(new_word);
+                           }
+                       }
+                   }
+               }
+           }
+           
+           for word in &level_visited {
+               word_set.remove(word);
+           }
+       }
+    
+       if !found {
+           return vec![];
+       }
+       
+       let mut result = Vec::new();
+       let mut path = vec![end_word.clone()];
+       
+       Self::dfs(&end_word, &begin_word, &parents, &mut path, &mut result);
+       
+       result
+   }
+   
+   fn dfs(word: &str, begin_word: &str, parents: &HashMap<String, Vec<String>>, 
+          path: &mut Vec<String>, result: &mut Vec<Vec<String>>) {
+       if word == begin_word {
+           let mut complete_path = path.clone();
+           complete_path.reverse();
+           result.push(complete_path);
+           return;
+       }
+       
+       if let Some(parent_words) = parents.get(word) {
+           for parent in parent_words {
+               path.push(parent.clone());
+               Self::dfs(parent, begin_word, parents, path, result);
+               path.pop(); 
+           }
+       }
+    }
+}