LeetCode 682: Baseball Game Solution in Python – A Step-by-Step Guide

What Is LeetCode 682: Baseball Game?

In LeetCode 682: Baseball Game, you’re given a list of strings operations representing a baseball game’s scoring moves, and your task is to calculate the total score. Each operation can be:

• An integer (e.g., "5"): Add points to the score.
• "+": Add the sum of the last two scores.
• "D": Double the last score and add it.
• "C": Cancel (remove) the last score.

You process the operations in order, maintaining a record of scores, and return the sum of all valid scores at the end. For example, with operations = ["5", "2", "C", "D", "+"], you’d score 5, then 2, cancel 2, double 5 to 10, and add 5+10=15, totaling 30. This problem tests your ability to manage a sequence of stack-like operations efficiently.

Problem Statement

• Input:
• operations: List of strings (integers or "C", "D", "+").
• Output: An integer—total score after all operations.
• Rules:
• Integer: Add to score.
• "C": Remove last score.
• "D": Double last score, add it.
• "+": Sum last two scores, add it.
• Process in order, sum all remaining scores.

Constraints

• 1 ≤ operations.length ≤ 1000.
• operations[i] is "C", "D", "+", or an integer in [-3 × 10⁴, 3 × 10⁴].
• Enough scores exist for "C", "D", "+" operations.

Examples

• Input: operations = ["5", "2", "C", "D", "+"]
• Output: 30 (5, 2, cancel 2, 10, 15 → 5 + 10 + 15 = 30)
• Input: operations = ["5", "-2", "4", "C", "D", "9", "+", "+"]
• Output: 27 (5, -2, 4, cancel 4, -4, 9, 5, 10 → 5 + (-4) + 9 + 5 + 10 = 25)
• Input: operations = ["1"]
• Output: 1 (Just 1)

These examples set the scorecard—let’s solve it!

Understanding the Problem: Tallying the Score

To solve LeetCode 682: Baseball Game in Python, we need to process a list of operations to compute the total score, maintaining a record of scores and applying rules for integers, "C", "D", and "+". A brute-force approach—recomputing sums repeatedly—would be inefficient. Since operations depend on previous scores in a last-in-first-out manner, a stack or array can track them efficiently. We’ll use:

• Best Solution (Stack with Operations): O(n) time, O(n) space—fast and intuitive.
• Alternative Solution (Array with Iteration): O(n) time, O(n) space—simple but less elegant.

Let’s start with the stack solution, breaking it down for beginners.

Best Solution: Stack with Operations

Why This Is the Best Solution

The stack with operations approach is the top choice for LeetCode 682 because it’s efficient—O(n) time with O(n) space—and naturally fits the problem’s last-in-first-out nature, using a stack to store scores and process operations directly. It fits constraints (operations.length ≤ 1000) perfectly and is intuitive once you see the stack logic. It’s like keeping a running tally where you can peek, pop, and push scores as the game unfolds!

How It Works

Think of this as a score stacker:

• Step 1: Initialize Stack:
• Empty stack to store scores.
• Step 2: Process Operations:
• For each op in operations:
• Integer: Convert to int, push to stack.
• "C": Pop last score from stack.
• "D": Peek last score, double it, push result.
• "+": Peek last two scores, sum them, push result.
• Step 3: Sum Stack:
• Sum all remaining scores in stack.
• Step 4: Return Result:
• Return total score.

It’s like a score keeper—stack and tally!

Step-by-Step Example

Example: operations = ["5", "2", "C", "D", "+"]

• Step 1: Stack = [].
• Step 2: Process:
• "5": Stack = [5].
• "2": Stack = [5, 2].
• "C": Pop 2, Stack = [5].
• "D": Peek 5, 5 * 2 = 10, Stack = [5, 10].
• "+": Peek 5, 10, 5 + 10 = 15, Stack = [5, 10, 15].
• Step 3: Sum = 5 + 10 + 15 = 30.
• Step 4: Return 30.
• Output: 30.

Example: operations = ["5", "-2", "4", "C", "D", "9", "+", "+"]

• Step 1: Stack = [].
• Step 2: Process:
• "5": Stack = [5].
• "-2": Stack = [5, -2].
• "4": Stack = [5, -2, 4].
• "C": Pop 4, Stack = [5, -2].
• "D": Peek -2, -2 * 2 = -4, Stack = [5, -2, -4].
• "9": Stack = [5, -2, -4, 9].
• "+": Peek -4, 9, -4 + 9 = 5, Stack = [5, -2, -4, 9, 5].
• "+": Peek 9, 5, 9 + 5 = 14, Stack = [5, -2, -4, 9, 5, 14].
• Step 3: Sum = 5 + (-2) + (-4) + 9 + 5 + 14 = 27.
• Step 4: Return 27.
• Output: 27.

Code with Detailed Line-by-Line Explanation

Here’s the Python code, explained clearly:

Copy codeclass Solution:
    def calPoints(self, operations: List[str]) -> int:
        # Step 1: Initialize stack
        stack = []

        # Step 2: Process each operation
        for op in operations:
            if op == "C":
                # Remove last score
                stack.pop()
            elif op == "D":
                # Double last score
                stack.append(stack[-1] * 2)
            elif op == "+":
                # Sum last two scores
                stack.append(stack[-1] + stack[-2])
            else:
                # Add integer score
                stack.append(int(op))

        # Step 3: Sum all scores in stack
        return sum(stack)

• Line 4: Init: Empty stack for scores.
• Lines 7-19: Process:
• "C": Pop last score.
• "D": Double last score, push.
• "+": Sum last two scores, push.
• Integer: Convert to int, push.
• Line 22: Return: Sum of stack.
• Time Complexity: O(n)—single pass, O(1) per operation.
• Space Complexity: O(n)—stack size.

This is like a score stacker—push and sum!

Alternative Solution: Array with Iteration

Why an Alternative Approach?

The array with iteration approach uses a list—O(n) time, O(n) space. It’s simpler conceptually, building the score list iteratively and summing at the end, but less dynamic than a stack due to indexing. It’s like keeping a flat scorecard and updating it step-by-step!

How It Works

Picture this as a score logger:

• Step 1: Init array for scores.
• Step 2: Process operations:
• Integer: Append to array.
• "C": Remove last element.
• "D": Double last element, append.
• "+": Sum last two elements, append.
• Step 3: Sum array.
• Step 4: Return total.

It’s like an array tallier—log and total!

Step-by-Step Example

Example: operations = ["5", "2", "C", "D", "+"]

• Step 1: Scores = [].
• Step 2: Process:
• "5": Scores = [5].
• "2": Scores = [5, 2].
• "C": Scores = [5].
• "D": Scores = [5, 10].
• "+": Scores = [5, 10, 15].
• Step 3: Sum = 5 + 10 + 15 = 30.
• Step 4: Return 30.
• Output: 30.

Code for Array Approach

Copy codeclass Solution:
    def calPoints(self, operations: List[str]) -> int:
        # Step 1: Initialize score array
        scores = []

        # Step 2: Process each operation
        for op in operations:
            if op == "C":
                scores.pop()
            elif op == "D":
                scores.append(scores[-1] * 2)
            elif op == "+":
                scores.append(scores[-1] + scores[-2])
            else:
                scores.append(int(op))

        # Step 3: Sum all scores
        total = 0
        for score in scores:
            total += score

        # Step 4: Return total
        return total

• Line 4: Init: Empty score list.
• Lines 7-15: Process:
• Same logic as stack, using array operations.
• Lines 18-20: Sum: Iterate to compute total.
• Line 23: Return total.
• Time Complexity: O(n)—single pass plus sum.
• Space Complexity: O(n)—array size.

It’s a score logger—list and add!

Comparing the Two Solutions

• Stack (Best):
• Pros: O(n) time, O(n) space, intuitive LIFO handling.
• Cons: Stack concept less obvious to beginners.
• Array (Alternative):
• Pros: O(n) time, O(n) space, simple iteration.
• Cons: Slightly less elegant, extra sum loop.

Stack wins for elegance.

Additional Examples and Edge Cases

• Input: operations = ["1"]
• Output: 1.
• Input: operations = ["5", "C"]
• Output: 0.

Stack handles these well.

Complexity Breakdown

• Stack: Time O(n), Space O(n).
• Array: Time O(n), Space O(n).

Stack is optimal for clarity.

Key Takeaways

• Stack: LIFO scoring—smart!
• Array: Linear logging—clear!
• Baseball: Tallying is fun.
• Python Tip: Stacks rock—see Python Basics.

Final Thoughts: Keep That Score

LeetCode 682: Baseball Game in Python is a fun scoring challenge. Stack with operations offers efficiency and elegance, while array with iteration provides a clear alternative. Want more? Try LeetCode 20: Valid Parentheses or LeetCode 155: Min Stack. Ready to tally? Head to Solve LeetCode 682 on LeetCode and calculate that score today!