648. Replace Words

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 rootforming 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

题目含义：英文中，以较短的单词为前缀，可以构成较长的单词。此时前缀可以称为“词根”。

给定一组词根字典dict，一个句子sentence。将句中的单词换为字典中出现过的最短词根。

思路：字典树（Trie）。利用词根dict构建字典树trie，遍历sentence中的word，在trie中进行搜索。

class Solution {
    
    class TrieNode {
        public char val;
        public boolean isWord;
        public TrieNode[] children = new TrieNode[26];
        public TrieNode() {}
        TrieNode(char c){
            val = c;
        }
    }    
    
    public String replaceWords(List<String> dict, String sentence) {
        String[] tokens = sentence.split(" ");
        TrieNode trie = buildTrie(dict);
        return replaceWords(tokens, trie);
    }

    private String replaceWords(String[] tokens, TrieNode root) {
        StringBuilder stringBuilder = new StringBuilder();
        for (String token : tokens) {
            stringBuilder.append(getShortestReplacement(token, root));
            stringBuilder.append(" ");
        }
        return stringBuilder.substring(0, stringBuilder.length()-1);
    }

    private String getShortestReplacement(String token, final TrieNode root) {
        TrieNode temp = root;
        StringBuilder stringBuilder = new StringBuilder();
        for (char c : token.toCharArray()) {
            stringBuilder.append(c);
            if (temp.children[c - 'a'] != null) {
                if (temp.children[c - 'a'].isWord) {
                    return stringBuilder.toString();
                }
                temp = temp.children[c - 'a'];
            } else {
                return token;
            }
        }
        return token;
    }

    private TrieNode buildTrie(List<String> dict) {
        TrieNode root = new TrieNode(' ');
        for (String word : dict) {
            TrieNode temp = root;
            for (char c : word.toCharArray()) {
                if (temp.children[c - 'a'] == null) {
                    temp.children[c - 'a'] = new TrieNode(c);
                }
                temp = temp.children[c - 'a'];
            }
            temp.isWord = true;
        }
        return root;
    }       
}