Clear Digits

Clear Digits

1. What is this problem about?

The Clear Digits interview question is a string manipulation challenge that involves a simple but iterative removal process. You are given a string that contains a mix of alphabetical characters and numerical digits. Your task is to repeatedly find a digit in the string and remove both that digit and the character immediately to its left. This process continues until no digits are left in the string. The challenge is to determine what the final string looks like after all such "clearing" operations have been performed.

2. Why is this asked in interviews?

Companies like Microsoft, Meta, and Google use the Clear Digits coding problem to test a candidate's understanding of the Simulation and Stack interview patterns. While it can be solved by repeatedly modifying the string, an efficient solution requires realizing that characters are removed in a Last-In-First-Out (LIFO) order relative to the digits. It evaluates whether you can avoid the $O(N^2)$ complexity of string re-allocation by using a more suitable data structure.

3. Algorithmic pattern used

The most efficient way to solve this is using a Stack. As you iterate through the string:

• If you encounter a letter, you push it onto the stack.
• If you encounter a digit, you pop the most recent letter from the stack. The stack effectively "remembers" the characters that are available to be cleared. After one pass through the string, the characters remaining in the stack form the final result.

4. Example explanation

Consider the string: abc12

1. First character is 'a' (letter): Add to stack. Stack: ['a']
2. Second character is 'b' (letter): Add to stack. Stack: ['a', 'b']
3. Third character is 'c' (letter): Add to stack. Stack: ['a', 'b', 'c']
4. Fourth character is '1' (digit): Remove the last letter ('c'). Stack: ['a', 'b']
5. Fifth character is '2' (digit): Remove the last letter ('b'). Stack: ['a'] Final result: "a".

5. Common mistakes candidates make

• Inefficient String Modification: Using a loop to find a digit and then slicing the string to remove characters. In many languages, strings are immutable, so this creates a new string every time, leading to poor performance.
• Handling Empty Stacks: Not considering what happens if a digit appears when there are no characters to its left (though problem constraints usually prevent this).
• Index Management: Trying to use a single pointer and manually adjusting it after removals, which is error-prone compared to a stack-based approach.

6. Interview preparation tip

Whenever a problem involves "removing an element based on a subsequent element," immediately think of a Stack. It’s the standard tool for matching pairs or handling nested structures. Practice similar problems like "Valid Parentheses" or "Backspace String Compare" to master this pattern.