The problem solved by this code is: we need to find a longest chain of substrings, where substring produced from string by dropping of one letter. The input reading is omitted.

longest_chain_of_substrings.cpp

#include <iostream>
#import <vector>
#import <string>
#import <set>
#import <algorithm>
#import <map>

using namespace std;

int main() {
    vector<string> w;
    sort(w.begin(), w.end(), [](const string& first, const string& second) {return first.size() < second.size();});
    set<string> s(w.begin(), w.end());
    map<string, set<string> > words;
    for (auto&& str: w) {
        for (unsigned j=0; j < str.length(); ++j) {
            string temp = str.substr(0, j) + str.substr(j + 1);
            if (s.find(temp) != s.end()) {
                words[str].insert(temp);
            }
        }
    }

    map<string, unsigned > maxlen;
    for (auto&& str: w) {
        unsigned maxl = 0;
        for (auto&&word: words[str]) {
            if (maxl < maxlen[word] + 1) {
                maxl = maxlen[word] + 1;
            }
        }
        if (maxl != 0) {
            maxlen[str] = maxl;
        } else {
            maxlen[str] = 0;
        }

    }
    auto pr = std::max_element(maxlen.begin(), maxlen.end(),
                               [](const pair<string, unsigned>& p1, const pair<string, unsigned>& p2) {
                                   return p1.second < p2.second; });
    cout << pr->first;
    return 0;
}