Cracking the Coding Interview, 4 Edition - 150 Programming Interview Questions and Solutions

Solutions to Chapter 20 | Hard
20.10 Given two words of equal length that are in a dictionary, write a method to transform
one word into another word by changing only one letter at a time. The new word you
get in each step must be in the dictionary.
EXAMPLE:
SOLUTION
Input: DAMP, LIKE
Output: DAMP -> LAMP -> LIMP -> LIME -> LIKE
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Though this problem seems tough, it’s actually a straightforward modification of breadthfirst-search.
Each word in our “graph” branches to all words in the dictionary that are one edit
away. The interesting part is how to implement this—should we build a graph as we go? We
could, but there’s an easier way. We can instead use a “backtrack map.” In this backtrack map,
if B[v] = w, then you know that you edited v to get w. When we reach our end word, we can
use this backtrack map repeatedly to reverse our path. See the code below:
1 LinkedList transform(String startWord, String stopWord,
2 Set dictionary) {
3 startWord = startWord.toUpperCase();
4 stopWord = stopWord.toUpperCase();
5 Queue actionQueue = new LinkedList();
6 Set visitedSet = new HashSet();
7 Map backtrackMap = new TreeMap();
8
9 actionQueue.add(startWord);
10 visitedSet.add(startWord);
11
12 while (!actionQueue.isEmpty()) {
13 String w = actionQueue.poll();
14 // For each possible word v from w with one edit operation
15 for (String v : getOneEditWords(w)) {
16 if (v.equals(stopWord)) {
17 // Found our word! Now, back track.
18 LinkedList list = new LinkedList();
19 // Append v to list
20 list.add(v);
21 while (w != null) {
22 list.add(0, w);
23 w = backtrackMap.get(w);
24 }
25 return list;
26 }
27 // If v is a dictionary word
2 9 1
Cracking the Coding Interview | Additional Review Problems