Introduction

In this blog post, we will explore how to solve the LeetCode problem "2019" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like Chatmagic, we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

Problem Statement

You are given a string s that contains digits 0-9, addition symbols '+', and multiplication symbols '' only, representing a valid math expression of single digit numbers (e.g., 3+52). This expression was given to n elementary school students. The students were instructed to get the answer of the expression by following this order of operations: Compute multiplication, reading from left to right; Then, Compute addition, reading from left to right. You are given an integer array answers of length n, which are the submitted answers of the students in no particular order. You are asked to grade the answers, by following these rules: If an answer equals the correct answer of the expression, this student will be rewarded 5 points; Otherwise, if the answer could be interpreted as if the student applied the operators in the wrong order but had correct arithmetic, this student will be rewarded 2 points; Otherwise, this student will be rewarded 0 points. Return the sum of the points of the students. Example 1: Input: s = "7+312", answers = [20,13,42] Output: 7 Explanation: As illustrated above, the correct answer of the expression is 13, therefore one student is rewarded 5 points: [20,13,42] A student might have applied the operators in this wrong order: ((7+3)1)2 = 20. Therefore one student is rewarded 2 points: [20,13,42] The points for the students are: [2,5,0]. The sum of the points is 2+5+0=7. Example 2: Input: s = "3+52", answers = [13,0,10,13,13,16,16] Output: 19 Explanation: The correct answer of the expression is 13, therefore three students are rewarded 5 points each: [13,0,10,13,13,16,16] A student might have applied the operators in this wrong order: ((3+5)2 = 16. Therefore two students are rewarded 2 points: [13,0,10,13,13,16,16] The points for the students are: [5,0,0,5,5,2,2]. The sum of the points is 5+0+0+5+5+2+2=19. Example 3: Input: s = "6+01", answers = [12,9,6,4,8,6] Output: 10 Explanation: The correct answer of the expression is 6. If a student had incorrectly done (6+0)1, the answer would also be 6. By the rules of grading, the students will still be rewarded 5 points (as they got the correct answer), not 2 points. The points for the students are: [0,0,5,0,0,5]. The sum of the points is 10. Constraints: 3 <= s.length <= 31 s represents a valid expression that contains only digits 0-9, '+', and '' only. All the integer operands in the expression are in the inclusive range [0, 9]. 1 <= The count of all operators ('+' and '') in the math expression <= 15 Test data are generated such that the correct answer of the expression is in the range of [0, 1000]. n == answers.length 1 <= n <= 104 0 <= answers[i] <= 1000

Explanation

Here's a breakdown of the approach, complexities, and the Python code:

• Key Idea: The problem requires calculating the correct answer based on the specified order of operations and then checking for alternative interpretations where addition precedes multiplication. We need to evaluate the expression twice: once with correct precedence and once with reversed precedence.
• Evaluation: Use stack-based evaluation to handle precedence correctly. For reversed precedence, we directly apply the operations from left to right without considering the order of operations.

• Grading: Iterate through the student answers and award points based on whether they match the correct answer or the alternative answer.

• Complexity:

  • Runtime: O(m + n), where m is the length of the string s and n is the length of the answers array.
  • Space: O(m) for the stacks used in evaluation.

Code

    def score_of_students(s: str, answers: list[int]) -> int:
    """
    Calculates the sum of points for students based on their answers to a math expression.

    Args:
        s: The math expression string.
        answers: A list of student answers.

    Returns:
        The sum of the points.
    """

    def calculate_correct(expression: str) -> int:
        """Calculates the correct answer based on precedence (multiplication before addition)."""
        nums = []
        ops = []
        i = 0
        while i < len(expression):
            if expression[i].isdigit():
                nums.append(int(expression[i]))
            elif expression[i] in ('+', '*'):
                ops.append(expression[i])
            i += 1

        i = 0
        while i < len(ops):
            if ops[i] == '*':
                nums[i] = nums[i] * nums[i + 1]
                nums.pop(i + 1)
                ops.pop(i)
                i -= 1
            i += 1

        result = nums[0]
        for i in range(len(ops)):
            if ops[i] == '+':
                result += nums[i + 1]
        return result

    def calculate_wrong(expression: str) -> int:
        """Calculates the answer with incorrect precedence (addition before multiplication)."""
        nums = []
        ops = []
        i = 0
        while i < len(expression):
            if expression[i].isdigit():
                nums.append(int(expression[i]))
            elif expression[i] in ('+', '*'):
                ops.append(expression[i])
            i += 1

        result = nums[0]
        for i in range(len(ops)):
            if ops[i] == '+':
                result += nums[i + 1]
            elif ops[i] == '*':
                result *= nums[i + 1]
        return result

    correct_answer = calculate_correct(s)
    wrong_answer = calculate_wrong(s)

    total_points = 0
    for answer in answers:
        if answer == correct_answer:
            total_points += 5
        elif answer == wrong_answer:
            total_points += 2

    return total_points