LeetCode 648. Replace Words

原题链接在这里：https://leetcode.com/problems/replace-words/description/

题目：

In English, we have a concept called root, which can be followed by some other words to form another longer word - let's call this word successor. For example, the root an, followed by other, which can form another word another.

Now, given a dictionary consisting of many roots and a sentence. You need to replace all the successor in the sentence with the root forming it. If a successor has many roots can form it, replace it with the root with the shortest length.

You need to output the sentence after the replacement.

Example 1:

Input: dict = ["cat", "bat", "rat"]
sentence = "the cattle was rattled by the battery"
Output: "the cat was rat by the bat"

Note:

1. The input will only have lower-case letters.
2. 1 <= dict words number <= 1000
3. 1 <= sentence words number <= 1000
4. 1 <= root length <= 100
5. 1 <= sentence words length <= 1000

题解：

利用Trie树. 把dict中每个词先放进Trie树中.

对于sentence按照空格拆分出的token, 从头添加token的char, 试验是否能在Trie树中找到. 找到说明加到这就是个root了, 可以返回.

若是Trie树中都没有starts with当前值时说明这个token 没有root, 返回原token.

Time Complexity: O(n*m + k*p). n = dict.size(). m是dict中单词平均长度. k是sentence中token个数. p是token平均长度.

Space: O(n*m+k).

AC Java:

class Solution {
    public String replaceWords(List<String> dict, String sentence) {
        if(sentence == null || sentence.length() == 0 || dict == null || dict.size() == 0){
            return sentence;
        }
        
        Trie trie = new Trie();
        for(String word : dict){
            trie.insert(word);
        }
        
        String [] tokens = sentence.split("\\s+");

        StringBuilder sb = new StringBuilder();
        for(String token : tokens){
            sb.append(getRootOrDefault(token, trie) + " ");
        }
        
        sb.deleteCharAt(sb.length()-1);
        return sb.toString();
    }
    
    private String getRootOrDefault(String s, Trie trie){
        StringBuilder sb = new StringBuilder();
        for(int i = 0; i<s.length(); i++){
            sb.append(s.charAt(i));
            if(trie.search(sb.toString())){
                return sb.toString();
            }else if(!trie.startsWith(sb.toString())){
                return s;
            }
        }
        return s;
    }
}

class Trie{
    private TrieNode root;
    
    public Trie(){
        root = new TrieNode();
    }
    
    public void insert(String word){
        TrieNode p = root;
        for(char c : word.toCharArray()){
            if(p.nexts[c-'a'] == null){
                p.nexts[c-'a'] = new TrieNode();
            }
            p = p.nexts[c-'a'];
        }
        
        p.val = word;
    }
    
    public boolean search(String word){
        TrieNode p = root;
        for(char c : word.toCharArray()){
            if(p.nexts[c-'a'] == null){
                return false;
            }
            p = p.nexts[c-'a'];
        }
        return p.val.equals(word);
    }
    
    public boolean startsWith(String word){
        TrieNode p = root;
        for(char c : word.toCharArray()){
            if(p.nexts[c-'a'] == null){
                return false;
            }
            p = p.nexts[c-'a'];
        }
        return true;
    }
}

class TrieNode{
    String val = "";
    TrieNode [] nexts;
    
    public TrieNode(){
        nexts = new TrieNode[26];
    }
}

Another implementation.

class Solution {
    public String replaceWords(List<String> dict, String sentence) {
        if(sentence == null || sentence.length() == 0 || dict == null || dict.size() == 0){
            return sentence;
        }
        
        TrieNode root = new TrieNode();
        for(String word : dict){
            TrieNode p = root;
            for(char c : word.toCharArray()){
                if(p.nexts[c-'a'] == null){
                    p.nexts[c-'a'] = new TrieNode();
                }
                
                p = p.nexts[c-'a'];
            }
            
            p.val = word;
        }
        
        String [] words = sentence.split("\\s+");
        for(int i = 0; i<words.length; i++){
            TrieNode p = root;
            for(char c : words[i].toCharArray()){
                if(p.nexts[c-'a'] == null || p.val != null){
                    break;
                }
                
                p = p.nexts[c-'a'];
            }
            
            words[i] = (p.val == null) ? words[i] : p.val;
        }
        
        return String.join(" ", words);
    }
}

class TrieNode{
    String val;
    TrieNode [] nexts;
    
    public TrieNode(){
        nexts = new TrieNode[26];
    }
}