LeetCode - Implement Magic Dictionary

Implement a magic directory with buildDict, and search methods.

For the method buildDict, you'll be given a list of non-repetitive words to build a dictionary.

For the method search, you'll be given a word, and judge whether if you modify exactly one character into another character in this word, the modified word is in the dictionary you just built.

Example 1:
Input: buildDict(["hello", "leetcode"]), Output: Null
Input: search("hello"), Output: False
Input: search("hhllo"), Output: True
Input: search("hell"), Output: False
Input: search("leetcoded"), Output: False
Note:
You may assume that all the inputs are consist of lowercase letters a-z.
For contest purpose, the test data is rather small by now. You could think about highly efficient algorithm after the contest.
Please remember to RESET your class variables declared in class MagicDictionary, as static/class variables are persisted across multiple test cases. Please see here for more details.

只需检测和要搜索单词长度一样的单词即可，所以我们用的数据结构就是根据单词的长度来分，把长度相同相同的单词放到一起，这样就可以减少搜索量。那么对于和要搜索单词进行比较的单词，由于已经保证了长度相等，我们直接进行逐个字符比较即可，用cnt表示不同字符的个数，初始化为0。如果当前遍历到的字符相等，则continue；如果当前遍历到的字符不相同，并且此时cnt已经为1了，则break，否则cnt就自增1。退出循环后，我们检测是否所有字符都比较完了且cnt为1，是的话则返回true，否则就是跟下一个词比较。如果所有词都比较完了，则返回false，参见代码如下：

class MagicDictionary {

    private HashMap<Integer, List<String>> map;

    /** Initialize your data structure here. */
    public MagicDictionary() {
        map = new HashMap<Integer, List<String>>();
    }

    /** Build a dictionary through a list of words */
    public void buildDict(String[] dict) {
        for(String str : dict){
            int len = str.length();
            if(map.containsKey(len)){
                map.get(len).add(str);
            }
            else{
                List<String> list = new ArrayList<>();
                list.add(str);
                map.put(len, list);
            }
        }
    }

    /** Returns if there is any word in the trie that equals to the given word after modifying exactly one character */
    public boolean search(String word) {
        if(word == null || word.length() == 0){
            return false;
        }
        int len = word.length();
        if(map.containsKey(len)){
            List<String> list = map.get(len);
            for(String str : list){
                if(hasExactOneDifference (str, word)){
                    return true;
                }
            }
        }
        return false;
    }

    private boolean hasExactOneDifference(String str1, String str2){
        int cnt = 0;
        for(int i = 0; i< str1.length(); i++){
            if(str1.charAt(i) != str2.charAt(i)){
                if(cnt == 1){
                    return false;
                }
                else{
                    cnt++;
                }
            }
        }
        if(cnt == 1){
            return true;
        }
        return false;

    }
}

/**
 * Your MagicDictionary object will be instantiated and called as such:
 * MagicDictionary obj = new MagicDictionary();
 * obj.buildDict(dict);
 * boolean param_2 = obj.search(word);
 */