g0101_0200.s0126_word_ladder_ii.Solution Maven / Gradle / Ivy

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package g0101_0200.s0126_word_ladder_ii;

// #Hard #String #Hash_Table #Breadth_First_Search #Backtracking
// #2022_07_02_Time_19_ms_(60.53%)_Space_43_MB_(90.12%)

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Queue;
import java.util.Set;

public class Solution {
    public List> findLadders(String beginWord, String endWord, List wordList) {
        List> ans = new ArrayList<>();
        // reverse graph start from endWord
        Map> reverse = new HashMap<>();
        // remove the duplicate words
        Set wordSet = new HashSet<>(wordList);
        // remove the first word to avoid cycle path
        wordSet.remove(beginWord);
        // store current layer nodes
        Queue queue = new LinkedList<>();
        // first layer has only beginWord
        queue.add(beginWord);
        // store nextLayer nodes
        Set nextLevel = new HashSet<>();
        // find endWord flag
        boolean findEnd = false;
        // traverse current layer nodes
        while (!queue.isEmpty()) {
            String word = queue.remove();
            for (String next : wordSet) {
                // is ladder words
                if (isLadder(word, next)) {
                    // construct the reverse graph from endWord
                    Set reverseLadders =
                            reverse.computeIfAbsent(next, k -> new HashSet<>());
                    reverseLadders.add(word);
                    if (endWord.equals(next)) {
                        findEnd = true;
                    }
                    // store next layer nodes
                    nextLevel.add(next);
                }
            }
            // when current layer is all visited
            if (queue.isEmpty()) {
                // if find the endWord, then break the while loop
                if (findEnd) {
                    break;
                }
                // add next layer nodes to queue
                queue.addAll(nextLevel);
                // remove all next layer nodes in wordSet
                wordSet.removeAll(nextLevel);
                nextLevel.clear();
            }
        }
        // if can't reach endWord from startWord, then return ans.
        if (!findEnd) {
            return ans;
        }
        Set path = new LinkedHashSet<>();
        path.add(endWord);
        // traverse reverse graph from endWord to beginWord
        findPath(endWord, beginWord, reverse, ans, path);
        return ans;
    }

    private void findPath(
            String endWord,
            String beginWord,
            Map> graph,
            List> ans,
            Set path) {
        Set next = graph.get(endWord);
        if (next == null) {
            return;
        }
        for (String word : next) {
            path.add(word);
            if (beginWord.equals(word)) {
                List shortestPath = new ArrayList<>(path);
                // reverse words in shortest path
                Collections.reverse(shortestPath);
                // add the shortest path to ans.
                ans.add(shortestPath);
            } else {
                findPath(word, beginWord, graph, ans, path);
            }
            path.remove(word);
        }
    }

    private boolean isLadder(String s, String t) {
        if (s.length() != t.length()) {
            return false;
        }
        int diffCount = 0;
        int n = s.length();
        for (int i = 0; i < n; i++) {
            if (s.charAt(i) != t.charAt(i)) {
                diffCount++;
            }
            if (diffCount > 1) {
                return false;
            }
        }
        return diffCount == 1;
    }
}