Introduction

In this blog post, we will explore how to solve the LeetCode problem "1441" 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 an integer array target and an integer n. You have an empty stack with the two following operations: "Push": pushes an integer to the top of the stack. "Pop": removes the integer on the top of the stack. You also have a stream of the integers in the range [1, n]. Use the two stack operations to make the numbers in the stack (from the bottom to the top) equal to target. You should follow the following rules: If the stream of the integers is not empty, pick the next integer from the stream and push it to the top of the stack. If the stack is not empty, pop the integer at the top of the stack. If, at any moment, the elements in the stack (from the bottom to the top) are equal to target, do not read new integers from the stream and do not do more operations on the stack. Return the stack operations needed to build target following the mentioned rules. If there are multiple valid answers, return any of them. Example 1: Input: target = [1,3], n = 3 Output: ["Push","Push","Pop","Push"] Explanation: Initially the stack s is empty. The last element is the top of the stack. Read 1 from the stream and push it to the stack. s = [1]. Read 2 from the stream and push it to the stack. s = [1,2]. Pop the integer on the top of the stack. s = [1]. Read 3 from the stream and push it to the stack. s = [1,3]. Example 2: Input: target = [1,2,3], n = 3 Output: ["Push","Push","Push"] Explanation: Initially the stack s is empty. The last element is the top of the stack. Read 1 from the stream and push it to the stack. s = [1]. Read 2 from the stream and push it to the stack. s = [1,2]. Read 3 from the stream and push it to the stack. s = [1,2,3]. Example 3: Input: target = [1,2], n = 4 Output: ["Push","Push"] Explanation: Initially the stack s is empty. The last element is the top of the stack. Read 1 from the stream and push it to the stack. s = [1]. Read 2 from the stream and push it to the stack. s = [1,2]. Since the stack (from the bottom to the top) is equal to target, we stop the stack operations. The answers that read integer 3 from the stream are not accepted. Constraints: 1 <= target.length <= 100 1 <= n <= 100 1 <= target[i] <= n target is strictly increasing.

Explanation

• Iterate through the target array: Maintain an index to track the current element in the target array that we are trying to achieve.
  • Simulate the stream: For each number from 1 to n, push it onto the stack. If the pushed number is not in the target array (and is less than the current target element), then pop it.
  • Stop when target is reached: Terminate the process when we have matched all elements in the target array.

• Time Complexity: O(n + m), where n is the upper bound of the stream and m is the length of the target array.
• Space Complexity: O(m), where m is the length of the target array, due to the list of operations stored.

Code

    def build_array(target: list[int], n: int) -> list[str]:
    """
    Builds a target array using push and pop operations from a stream of integers.

    Args:
        target: The target array to build.
        n: The upper bound of the integer stream.

    Returns:
        A list of strings representing the push and pop operations.
    """

    operations = []
    target_index = 0
    for i in range(1, n + 1):
        operations.append("Push")
        if i != target[target_index]:
            operations.append("Pop")
        else:
            target_index += 1
            if target_index == len(target):
                break
    return operations