Word Ladder

Given two words (beginWord and endWord), and a dictionary, find the length of 

shortest transformation sequence from beginWord to endWord, such that:

Only one letter can be changed at a time

Each intermediate word must exist in the dictionary

For example,

Given:start = "hit"  end = "cog"  dict = ["hot","dot","dog","lot","log"]

As one shortest transformation is "hit" -> "hot" -> "dot" -> "dog" -> "cog",

return its length 5.

Note:

Return 0 if there is no such transformation sequence.

All words have the same length.

All words contain only lowercase alphabetic characters.

从前向后将字符串的字母替换，直至最后得到想要的字符串。

public int ladderLength(String start, String end, Set<String> dict) {if (dict == null || dict.size() == 0) return 0;Queue<String> queue = new LinkedList<String>();queue.offer(start);dict.remove(start);int length = 1;while(!queue.isEmpty()) {int count = queue.size();for (int i = 0; i<count; i++){String current = queue.poll();for (char c = 'a'; c <= 'z'; c++) {for (int j=0; j < current.length(); j++) {if (c == current.charAt(j)) continue;String tmp = replace(current, j, c);if (tmp.equals(end)) return length + 1;if (dict.contains(tmp)){queue.offer(tmp);dict.remove(tmp);}}}}length++;}return 0;}private String replace(String s, int index, char c) {char[] chars = s.toCharArray();chars[index] = c;return new String(chars);}