使用map能直接用索引來比對，這題有點階梯感(? 比如說你跨上一個階梯後，要往下踩一格踏踏看有沒有穩，才能繼續往上踩(雖然很抽象，但我腦袋就浮出這些東西><)

class Solution {
public:
    static bool cmp(const string &s1, const string &s2){
        return s1.length() < s2.length();
    }
    int longestStrChain(vector<string>& words) {
        sort(words.begin(), words.end(), cmp);
        unordered_map<string, int> ump;
        int ans = 0;
        for(string w : words){
            int longest = 0;
            for(int i = 0; i < w.length(); i++){
                string sub = w;
                sub.erase(i, 1);
                longest = max(longest, ump[sub] + 1);
            }
            ump[w] = longest;
            ans = max(ans, longest);
        }
        return ans;
    }
};