function solveClasses() {
    class Person {
        constructor(firstName, lastName) {
            this.firstName = firstName;
            this.lastName = lastName;
            this.problems = [];
        }

        toString() {
            return `${this.firstName} ${this.lastName} is part of SoftUni community now!`
        }
    }

    class Teacher extends Person {
        constructor(firstName, lastName) {
            super(firstName, lastName);
        }

        createProblem(id, difficulty) {
            let newProblem = {
                id,
                difficulty
            }
            this.problems.push(newProblem);
            return this.problems;
        }

        getProblem() {
            return this.problems;
        }

        showProblemSolution(id) {
            let check = this.problems.find(p => p.id === id);
            if (!check) {
                throw new Error(`Problem with id ${id} not found.`)

            } else {
                check.difficulty -= 1;
                return check;
            }
        }
    }

    class Student extends Person {
        constructor(firstName, lastName, graduationCredits, problems) {
            super(firstName, lastName);
            this.graduationCredits = graduationCredits;
            this.myCredits = 0;
            this.solvedProblems = [];
            this.problems=problems;
        }

        solveProblem(id) {
            let check = this.problems.find(p => p.id === id);
            if (!check) {
                throw new Error(`Problem with id ${id} not found.`)
            } else if (!this.solvedProblems.includes(check)) {
                this.solvedProblems.push(check);
                this.myCredits += check.difficulty;
            }

            return this.myCredits;
        }

        graduate() {
            if (this.myCredits >= this.graduationCredits) {
                return `${this.firstName} ${this.lastName} has graduated succesfully.`
            } else {
                return `${this.firstName} ${this.lastName}, you need ${this.graduationCredits - this.myCredits} credits to graduate.`
            }
        }
    }
    return {
        Person,
        Teacher,
        Student
    }
}