Introduction

In this blog post, we will explore how to solve the LeetCode problem "1249" 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

Given a string s of '(' , ')' and lowercase English characters. Your task is to remove the minimum number of parentheses ( '(' or ')', in any positions ) so that the resulting parentheses string is valid and return any valid string. Formally, a parentheses string is valid if and only if: It is the empty string, contains only lowercase characters, or It can be written as AB (A concatenated with B), where A and B are valid strings, or It can be written as (A), where A is a valid string. Example 1: Input: s = "lee(t(c)o)de)" Output: "lee(t(c)o)de" Explanation: "lee(t(co)de)" , "lee(t(c)ode)" would also be accepted. Example 2: Input: s = "a)b(c)d" Output: "ab(c)d" Example 3: Input: s = "))((" Output: "" Explanation: An empty string is also valid. Constraints: 1 <= s.length <= 105 s[i] is either '(' , ')', or lowercase English letter.

Explanation

Here's the breakdown of the approach and the Python code:

• Identify Unmatched Parentheses: Iterate through the string to track the balance of open and close parentheses. Use a stack or counter to identify the indices of unmatched parentheses.
• Mark Invalid Parentheses: Mark the invalid parentheses (those that need to be removed) using a boolean array or by directly modifying the string (e.g., replacing them with an empty string placeholder).

• Build the Result: Construct the final valid string by iterating through the original string (or the modified string with placeholders) and including only the characters that are not marked as invalid.

• Time and Space Complexity: O(n) time complexity, O(n) space complexity.

Code

    def minRemoveToMakeValid(s: str) -> str:
    """
    Removes the minimum number of parentheses from a string to make it valid.

    Args:
        s: The input string containing '(', ')', and lowercase English characters.

    Returns:
        A valid string with the minimum number of parentheses removed.
    """

    n = len(s)
    indices_to_remove = set()
    stack = []

    for i, char in enumerate(s):
        if char == '(':
            stack.append(i)
        elif char == ')':
            if stack:
                stack.pop()
            else:
                indices_to_remove.add(i)

    indices_to_remove.update(stack)  # Add any remaining open parentheses

    result = ""
    for i, char in enumerate(s):
        if i not in indices_to_remove:
            result += char

    return result