2264. Largest 3-Same-Digit Number in String

Time: O(10 * n)

Space: O(1)

Intuition

Remove leading zeros from non-first digits. For number with k zeros before non-zero: remove k-1 zeros (keep one). Special case: all zeros.

Algorithm

1Split by non-zero digits. For each group of zeros between digits: keep exactly one zero.
2Or: regex replace leading zeros in each numeric group.

Common Pitfalls

•First digit cannot be zero. Inner groups can have one zero. Trailing zeros stay.

2264.cs

// Approach: Scan from digit '9' down; return first three-identical-digit substring found.
// Time: O(10 * n) Space: O(1)

public class Solution
{
    public string LargestGoodInteger(string num)
    {
        // Loop backwards from '9' to '0'
        // This will ensure that the first good integer found is the largest
        for (int i = 9; i >= 0; i--)
        {
            // Create a string consisting of three identical digits
            string tripleDigit = new string(char.Parse(i.ToString()), 3);

            // Check if the num string contains the tripleDigit
            if (num.Contains(tripleDigit))
                // Return the found good integer (since we started from 9, it's the largest)
                return tripleDigit;
        }

        // If no good integer is found, return an empty string
        return "";
    }
}

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