Hard 单词变型成另一个单词 @CareerCup

BFS的变形体。对起始单词的每一个字母都做替换，把替换后的单词加入到queue中，并同时维护一个Map来记录变化前单词和变化后单词的联系。用来回溯时能打印出路径。

package Hard;import java.util.HashSet;import java.util.LinkedList;import java.util.Map;import java.util.Queue;import java.util.Set;import java.util.TreeMap;import java.util.TreeSet;/*** Given two words of equal length that are in a dictionary, write a method to trans-form one word into another word by changing only one letter at a time. The newword you get in each step must be in the dictionary.给定两个有着相同长度且都在字典内的单词，要求写一个方法来把一个单词变型成另一个单词。一次只能转换一个字母，且每次生成的单词必须在字典内**/public class S18_10 {public static LinkedList<String> transform(String startWord, String stopWord, Set<String> dictionary) {startWord = startWord.toUpperCase();stopWord = stopWord.toUpperCase();Queue<String> actionQueue = new LinkedList<String>();Set<String> visitedSet = new HashSet<String>();Map<String, String> backtrackMap = new TreeMap<String, String>();actionQueue.add(startWord);visitedSet.add(startWord);while (!actionQueue.isEmpty()) {String w = actionQueue.poll();// For each possible word v from w with one edit operationfor (String v : getOneEditWords(w)) {if (v.equals(stopWord)) { // Found our word! Now, back track.LinkedList<String> list = new LinkedList<String>();list.add(v); // Append v to listwhile (w != null) {list.add(0, w);w = backtrackMap.get(w);}return list;}// If v is a dictionary wordif (dictionary.contains(v)) {if (!visitedSet.contains(v)) {actionQueue.add(v);visitedSet.add(v); // mark visitedbacktrackMap.put(v, w); // w is the previous state of v}}}}return null;}// 改变word的某一个字母private static Set<String> getOneEditWords(String word) {Set<String> words = new TreeSet<String>();for (int i = 0; i < word.length(); i++) { // for every letterchar[] wordArray = word.toCharArray();// change that letter to something elsefor (char c = 'A'; c <= 'Z'; c++) {if (c != word.charAt(i)) {wordArray[i] = c;words.add(new String(wordArray));}}}return words;}public static HashSet<String> setupDictionary(String[] words) {HashSet<String> hash = new HashSet<String>();for (String word : words) {hash.add(word.toUpperCase());}return hash;}public static void main(String[] args) {String[] words = { "maps", "tan", "tree", "apple", "cans", "help","aped", "free", "apes", "flat", "trap", "fret", "trip", "trie","frat", "fril" };HashSet<String> dict = setupDictionary(words);LinkedList<String> list = transform("tree", "flat", dict);for (String word : list) {System.out.println(word);}}}