[Leetcode] 127. Word Ladder

Given two words (start and end), and a dictionary, find the length of shortest transformation sequence from start to end, such that:

1. Only one letter can be changed at a time
2. 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.

import java.util.HashSet;import java.util.LinkedList;public class Solution {    public int ladderLength(String start, String end, Set<String> dict) {        if(start == null || end == null || 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 size = queue.size();            for(int i = 0; i < size; 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 temp = replace(current, j, c);                        if(temp.equals(end)){                            return length + 1;                        }                        if(dict.contains(temp)){                            queue.offer(temp);                            dict.remove(temp);                        }                    }                }            }            length++;        }        return 0;    }    private String replace(String s, int index, char c) {        char[] chars = s.toCharArray();        chars[index] = c;        return new String(chars);    }}