1048. Longest String Chain

https://leetcode.com/problems/longest-string-chain/discuss/1543825/Java-DP-with-HashMap-clear-explanation

DP with HashMap

1. from shortest words, words[] sort by length

2. dp with hashmap

because this question want find longest word chain with predecessor, 
so we can think about use map to store word predecessor

3. for each word

ex1
Input: words = ["a","b","ba","bca","bda","bdca"]
Output: 4

"a",       the longest word chain: 1, save to map(a, 1)
"b",       the longest word chain: 1, save to map(b, 1)

"ba", => 
we can remove one char to find in map, is there a predecessor, 

so remove b => map.get(a) = 1, so 1 + 1(remove one char) = 2
remove a  => map.get(b) = 1, so 1 + 1 = 2, store map<ba, 2>

"bca",  remove c, get(ba) = 2 + 1 = 3 
"bda",  remove d, get(ba) = 2 + 1 = 3 
"bdca", remove d, get(bca) = 3 + 1 = 4

ans is 4

why count = Math.max(count, map.get(findStr)+1); needs do max?

["bdca","bda","ca","dca","a"]
ans: 4

a, ca, dca, bda, bdca

a 1
ca 2
dca 3
bda 1
bdca 4 => bda =>(1+1), dca(3+1), so when cal count, should take max count result

if (map.containsKey(findStr)) {
    count = Math.max(count, map.get(findStr)+1);
}

time: O(nlogn + n*w), n is words[i].length (1~16) , w is word length(1~1000)

space: O(n)

class Solution {
    public int longestStrChain(String[] words) {
        int res = 0;
        
        // dp means when word = xxx, word's the longest predecessor number: 
        // map(word, the longest predecessor number)
        Map<String, Integer> map = new HashMap<>(); 
        
        // notice this sort by string len
        // needs sort asc, because start from short one is easy to make longer word chain
        Arrays.sort(words, (a, b) -> a.length() - b.length());
        
        for (String word : words) {
            int curr = 1; // init each word's predecessor is 1
            for (int i = 0; i < word.length(); i++) {
                StringBuilder sb = new StringBuilder(word); 
                
                // remove one char in word, to find if there is a larger predecessor in map
                String next = sb.deleteCharAt(i).toString(); 
                
                if (map.containsKey(next)) {
                    curr = Math.max(map.get(next) + 1,  curr);
                }
            }
            map.put(word, curr); // update the map(word, longer value)
            res = Math.max(res, curr); // keep longer value
        }
        return res;
    }
}