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Largest 3-Same-Digit Number in String

Difficulty: Easy

Problem Description

You are given a string num representing a large integer. An integer is good if it meets the following conditions:

  • It is a substring of num with length 3.
  • It consists of only one unique digit.

Return the maximum good integer as a string or an empty string if no such integer exists.

Key Insights

  • A good integer must be exactly three characters long.
  • Each good integer consists of the same digit repeated three times.
  • We can iterate through the string and check each substring of length 3.
  • We need to keep track of the largest good integer found during the iteration.

Space and Time Complexity

Time Complexity: O(n) - where n is the length of the string num, as we make a single pass through the string. Space Complexity: O(1) - we only store a few variables for tracking the maximum good integer.

Solution

To solve the problem, we can use a simple loop to iterate through the string while checking each substring of length 3. We will check if all three characters are the same and track the maximum good integer found. If no good integers are found, we will return an empty string at the end.

Code Solutions

def largest_good_integer(num: str) -> str:
    max_good_integer = ""
    
    for i in range(len(num) - 2):
        substring = num[i:i + 3]
        if substring[0] == substring[1] == substring[2]:  # Check if all digits are the same
            if substring > max_good_integer:  # Compare lexicographically
                max_good_integer = substring
                
    return max_good_integer
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