Word Ladder

public class Solution {
    public int ladderLength(String beginWord, String endWord, Set<String> wordList) {
        if(!isValidParam(beginWord, endWord, wordList)) {
            return 0;
        } else if(beginWord.equals(endWord)) {
            return 1;
        }
        
        Set<String> visited = new HashSet<>();
        LinkedList<String> currLevel = new LinkedList<>();
        LinkedList<String> nextLevel = new LinkedList<>();
        int cnt = 1;
        currLevel.offer(beginWord);
        visited.add(beginWord);
        
        while(currLevel.size() > 0) {
            for(String word: currLevel) {
                StringBuilder buff = new StringBuilder(word);
                for(int i = 0; i < buff.length(); i++) {
                    char currChar = buff.charAt(i);
                    for(char c = 'a'; c <= 'z'; c++) {
                        buff.setCharAt(i, c);
                        String tmpWord = buff.toString();
                        
                        // found the endWord
                        if(endWord.equals(tmpWord)) {
                            return cnt + 1;
                        }
                        
                        if(visited.contains(tmpWord) || !wordList.contains(tmpWord)) {
                            continue;
                        } else {
                            visited.add(tmpWord);
                            nextLevel.add(tmpWord);
                        }
                    }
                    buff.setCharAt(i, currChar);
                }
            }
            currLevel = nextLevel;
            nextLevel = new LinkedList<String>();
            cnt++;
        }
        
        return 0;
    }
    
    private boolean isValidParam(String beginWord, String endWord, Set<String> dict) {
        boolean isValid = true;
        if(beginWord == null || endWord == null || beginWord.length() != endWord.length()) {
            isValid = false;
        }
        if(dict == null || dict.size() == 0) {
            isValid = false;
        }
        return isValid;
    }
}