LeetCode 1048. Longest String Chain

原題連接在這裏：https://leetcode.com/problems/longest-string-chain/

題目：

Given a list of words, each word consists of English lowercase letters.

Let's say word1 is a predecessor of word2 if and only if we can add exactly one letter anywhere in word1 to make it equal to word2.  For example, "abc" is a predecessor of "abac".

A word chain is a sequence of words [word_1, word_2, ..., word_k] with k >= 1, where word_1 is a predecessor of word_2, word_2 is a predecessor of word_3, and so on.

Return the longest possible length of a word chain with words chosen from the given list of words.

Example 1:

Input: ["a","b","ba","bca","bda","bdca"]
Output: 4 Explanation: one of the longest word chain is "a","ba","bda","bdca".

Note:

1. 1 <= words.length <= 1000
2. 1 <= words[i].length <= 16
3. words[i] only consists of English lowercase letters.

題解：

Sort the words based on word length.

For each word, find all its possible predecessors by deleting one char from the word, if it exist, update longest chain length up to this word. 

And put it in the mapping.

At the same time, update the global longest res.

Time Complexity: O(nlogn + n*len). n is the words count. len is the average length of word.

Space: O(n).

AC Java:

 1 class Solution {
 2     public int longestStrChain(String[] words) {
 3         HashMap<String, Integer> hm = new HashMap<>();
 4         Arrays.sort(words, (a, b) -> a.length() - b.length());
 5         
 6         int res = 0;
 7         for(String word : words){
 8             int best = 0;
 9             for(int i = 0; i<word.length(); i++){
10                 String prev = word.substring(0, i) + word.substring(i+1);
11                 best = Math.max(best, hm.getOrDefault(prev, 0)+1);
12             }
13             
14             hm.put(word, best);
15             res = Math.max(res, best);
16         }
17         
18         return res;
19     }
20 }