LeetCode: Clear Digits

Problem:

https://leetcode.com/problems/clear-digits/description/

Code:

class Solution:
    def clearDigits(self, s: str) -> str:
        stack=[]
        for ch in s:
            if not ch.isdigit():
                stack.append(ch)
            else:
                if stack:
                    stack.pop()
        return "".join(stack)

Key Points:

Core Idea (Use Stack to Remove Characters)

We scan the string from left → right.

• When we see a letter → push it into the stack (keep it).

• When we see a digit → delete the most recent letter by popping the stack.

Digits do not stay in the final answer. They only trigger removal of a previous character.

How It Works Step-by-Step

For each character ch in the string:

• If ch is NOT a digit: → add it to stack (it’s a valid character)

• If ch IS a digit: → remove the last character from stack (if stack not empty)

After processing all characters:

• Stack contains only the remaining letters in correct order

• Convert list → string using "".join(stack)

Why Use a Stack?

• Stack naturally follows Last In, First Out behavior

• The most recent valid character is removed first by digits

• This exactly matches the problem rule

Edge Cases Covered

• If a digit appears and stack is empty → ignore (nothing to delete)

• Multiple digits remove multiple previous characters

• Only letters remain in final output

Complexity

Time: O(n) → process each character once
Space: O(n) → stack may store up to all letters