Is Apply Operations to Make String Empty easy or hard?

The problem is rated Medium because the direct simulation is easy to understand but inefficient to reason about. The challenge is recognizing the frequency pattern that determines which characters survive until the final round.

Apply Operations to Make String Empty Python/Java solution

In Python or Java, the typical implementation counts character frequencies using an array or map, finds the maximum frequency, then collects characters whose last occurrence corresponds to that frequency layer. The algorithm runs in linear time and is easy to implement with standard string iteration.

How to solve Apply Operations to Make String Empty in O(n)?

Count how many times each character appears in the string. Find the maximum frequency, then scan the string and track the last occurrence of characters with that frequency. Sorting those characters by their last index produces the final string that remains before the last removal round.

What is the best approach for Apply Operations to Make String Empty?

The optimal approach uses frequency counting to determine which characters survive the most rounds. Characters with the maximum frequency remain until the final operation, so the answer is formed from their last occurrences in the string. This method runs in O(n) time with O(1) extra space because the alphabet size is fixed.

Is Apply Operations to Make String Empty asked at Google/Amazon/Meta?

Problems involving string frequency analysis and layered removals are common in interviews at companies like Amazon, Google, and Meta. Variants of this question test understanding of counting techniques, simulation reduction, and recognizing patterns that eliminate repeated operations.

What data structure is used in Apply Operations to Make String Empty?

The main data structures are a counting array or hash table for tracking character frequencies and sometimes queues when simulating the process directly. The optimized approach relies mostly on arrays and index tracking rather than complex structures.

What is the time complexity of Apply Operations to Make String Empty?

The optimal solution runs in O(n) time where n is the length of the string. You perform one pass to count character frequencies and another pass to identify the final surviving characters. Space complexity is O(1) using a fixed-size counting array for lowercase letters.

Description Hints 2 Solution Video 8:59

3039. Apply Operations to Make String Empty

Medium57.5% acceptance1 company

You are given a string s.

Consider performing the following operation until s becomes empty:

For every alphabet character from 'a' to 'z', remove the first occurrence of that character in s (if it exists).

For example, let initially s = "aabcbbca". We do the following operations:

Remove the underlined characters s = "aabcbbca". The resulting string is s = "abbca".
Remove the underlined characters s = "abbca". The resulting string is s = "ba".
Remove the underlined characters s = "ba". The resulting string is s = "".

Return the value of the string s right before applying the last operation. In the example above, answer is "ba".

Example 1:

Input: s = "aabcbbca"
Output: "ba"
Explanation: Explained in the statement.

Example 2:

Input: s = "abcd"
Output: "abcd"
Explanation: We do the following operation:
- Remove the underlined characters s = "abcd". The resulting string is s = "".
The string just before the last operation is "abcd".

Constraints:

1 <= s.length <= 5 * 10⁵
s consists only of lowercase English letters.

Asked at 1 company

Virtusa

Topics

Hints open one at a time

Hint 1

Before the last operation, only the most frequent characters in the original string will remain.

Hint 2

Keep only the last occurence of each of the most frequent characters.

8:59

Still stuck?Read 5 approaches, or watch a walkthrough.

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1class Solution:
2    def lastNonEmptyString(self, s: str) -> str:
3        

Starter codePython 3Spaces: 4

Examples

Case 1Case 2

Input

"aabcbbca"

Input

"abcd"

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