3174. Clear Digits

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String Simulation

Time: O(n)

Space: O(n)

Intuition

Clear digits: each digit removes the first non-digit to its left. Stack simulation.

Algorithm

1Stack. For each char: if digit, pop top (if non-empty). Else push char.

Common Pitfalls

•Digit removes nearest non-digit to its left. Stack handles LIFO order naturally.

3174.cs

// Approach: Stack; remove each digit and the nearest preceding non-digit character.
// Time: O(n) Space: O(n)

public class Solution
{
    public string ClearDigits(string s)
    {
        StringBuilder sb = new StringBuilder();

        foreach (char c in s)
        {
            if (char.IsDigit(c))
            {
                // Since `sb` only contains non-digit characters, popping the last
                // character is equivalent to deleting the closest non-digit character.
                if (sb.Length > 0)
                    sb.Length--;
            }
            else
                sb.Append(c);
        }

        return sb.ToString();
    }
}

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