Is Remove Palindromic Subsequences easy or hard?

Remove Palindromic Subsequences is classified as an Easy problem on LeetCode with a high acceptance rate. The challenge lies in recognizing the key observation that only two operations are ever needed due to the limited character set.

Remove Palindromic Subsequences Python/Java solution

Most implementations check whether the string equals its reverse (Python) or use two pointers moving inward (Java/C++). If the string is empty return 0, if it is a palindrome return 1, otherwise return 2. The logic is simple and runs in linear time.

How to solve Remove Palindromic Subsequences in O(n)?

Traverse the string using two pointers from both ends. If all mirrored characters match, the string is a palindrome and the answer is 1. If a mismatch appears, the answer becomes 2 because you can remove all 'a' characters in one operation and all 'b' characters in another. The scan touches each character once, giving O(n) time complexity.

What is the best approach for Remove Palindromic Subsequences?

The optimal approach checks whether the entire string is already a palindrome using the two pointers technique. If the string is a palindrome, it can be removed in one operation. Otherwise, remove all 'a' characters in one step and all 'b' characters in another, resulting in at most two operations. This solution runs in O(n) time and O(1) space.

Is Remove Palindromic Subsequences asked at Google/Amazon/Meta?

Palindrome and string manipulation problems appear frequently in interviews at companies like Amazon, Google, and Meta. While this exact problem may not always appear, its core ideas—two pointers and palindrome checks—are common interview patterns.

What data structure is used in Remove Palindromic Subsequences?

The main technique relies on two pointers for palindrome verification. Some explanations also use a small set or frequency counter to track unique characters in the string. Both approaches operate with constant extra memory.

What is the time complexity of Remove Palindromic Subsequences?

The optimal solution runs in O(n) time where n is the length of the string. You perform a single pass to check whether the string is a palindrome or to inspect the characters present. Space complexity remains O(1) because only a few pointers or counters are required.

Description Hints 2 Solution Video 4:25

1332. Remove Palindromic Subsequences

Easy77.1% acceptance1 company

You are given a string s consisting only of letters 'a' and 'b'. In a single step you can remove one palindromic subsequence from s.

Return the minimum number of steps to make the given string empty.

A string is a subsequence of a given string if it is generated by deleting some characters of a given string without changing its order. Note that a subsequence does not necessarily need to be contiguous.

A string is called palindrome if is one that reads the same backward as well as forward.

Example 1:

Input: s = "ababa"
Output: 1
Explanation: s is already a palindrome, so its entirety can be removed in a single step.

Example 2:

Input: s = "abb"
Output: 2
Explanation: "abb" -> "bb" -> "". 
Remove palindromic subsequence "a" then "bb".

Example 3:

Input: s = "baabb"
Output: 2
Explanation: "baabb" -> "b" -> "". 
Remove palindromic subsequence "baab" then "b".

Constraints:

1 <= s.length <= 1000
s[i] is either 'a' or 'b'.

Asked at 1 company

Amazon

Topics

String Two Pointers

Hints open one at a time

Hint 1

Use the fact that string contains only 2 characters.

Hint 2

Are subsequences composed of only one type of letter always palindrome strings ?

4:25

Still stuck?Read 3 approaches, or watch a walkthrough.

Open solution

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

Starter codePython 3Spaces: 4

Examples

Case 1Case 2Case 3

Input

"ababa"

Input

"abb"

Input

"baabb"

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