Task: Abbreviations, automatic
C++: #include <iomanip>
#include <iostream>
#include <fstream>
#include <map>
#include <sstream>
#include <string>
#include <vector>

std::vector<std::string> split(const std::string& str, char delimiter) {
    std::vector<std::string> tokens;
    std::string token;
    std::istringstream tokenStream(str);
    while (std::getline(tokenStream, token, delimiter)) {
        tokens.push_back(token);
    }
    return tokens;
}

int main() {
    using namespace std;
    string line;
    int i = 0;

    ifstream in("days_of_week.txt");
    if (in.is_open()) {
        while (getline(in, line)) {
            i++;
            if (line.empty()) {
                continue;
            }

            auto days = split(line, ' ');
            if (days.size() != 7) {
                throw std::runtime_error("There aren't 7 days in line " + i);
            }

            map<string, int> temp;
            for (auto& day : days) {
                if (temp.find(day) != temp.end()) {
                    cerr << " ∞  " << line << '\n';
                    continue;
                }
                temp[day] = 1;
            }

            int len = 1;
            while (true) {
                temp.clear();
                for (auto& day : days) {
                    string key = day.substr(0, len);
                    if (temp.find(key) != temp.end()) {
                        break;
                    }
                    temp[key] = 1;
                }
                if (temp.size() == 7) {
                    cout << setw(2) << len << "  " << line << '\n';
                    break;
                }
                len++;
            }
        }
    }

    return 0;
}

Java: import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Abbreviations {
    public static void main(String[] args) throws IOException {
        Path path = Paths.get("days_of_week.txt");
        List<String> readAllLines = Files.readAllLines(path);
        for (int i = 0; i < readAllLines.size(); i++) {
            String line = readAllLines.get(i);
            if (line.length() == 0) continue;

            String[] days = line.split(" ");
            if (days.length != 7) throw new RuntimeException("There aren't 7 days on line " + (i + 1));

            Map<String, Integer> temp = new HashMap<>();
            for (String day : days) {
                Integer count = temp.getOrDefault(day, 0);
                temp.put(day, count + 1);
            }
            if (temp.size() < 7) {
                System.out.print(" ∞  ");
                System.out.println(line);
                continue;
            }

            int len = 1;
            while (true) {
                temp.clear();
                for (String day : days) {
                    String sd;
                    if (len >= day.length()) {
                        sd = day;
                    } else {
                        sd = day.substring(0, len);
                    }
                    Integer count = temp.getOrDefault(sd, 0);
                    temp.put(sd, count + 1);
                }
                if (temp.size() == 7) {
                    System.out.printf("%2d  %s\n", len, line);
                    break;
                }
                len++;
            }
        }
    }
}