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Second Largest Digit in a String

Difficulty: Easy

Problem Description

Given an alphanumeric string s, return the second largest numerical digit that appears in s, or -1 if it does not exist. An alphanumeric string is a string consisting of lowercase English letters and digits.

Key Insights

  • The problem requires identifying numerical digits within a string.
  • We need to find the unique digits to determine the second largest.
  • If there are fewer than two unique digits, the result should be -1.

Space and Time Complexity

Time Complexity: O(n), where n is the length of the string, since we need to iterate through the string once. Space Complexity: O(1), as the space used for storing digits is constant (only 10 possible digits).

Solution

To solve this problem, we can:

  1. Initialize a set to store unique digits found in the string.
  2. Iterate through each character in the string, checking if it is a digit.
  3. If it is a digit, add it to the set.
  4. Convert the set to a sorted list to find the second largest digit.
  5. Return the second largest digit if it exists; otherwise, return -1.

Code Solutions

def second_largest_digit(s: str) -> int:
    # Use a set to store unique digits
    digits = set()
    
    # Iterate through each character in the string
    for char in s:
        if char.isdigit():
            # Add the digit to the set
            digits.add(int(char))
    
    # Convert the set to a sorted list
    sorted_digits = sorted(digits)
    
    # Check if there are at least two unique digits
    if len(sorted_digits) < 2:
        return -1
    
    # Return the second largest digit
    return sorted_digits[-2]
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