1332. Remove Palindromic Subsequences

Description

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'.

💡 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?

Submitted Code

class Solution:
    def removePalindromeSub(self, s: str) -> int:
        if s == "":         # empty
            return 0
        if s == s[::-1]:    # already a palindrome
            return 1
        return 2

Runtime: 0 ms | Beats 100.00%
Memory: 19.28 MB | Beats 70.00%

제시된 예시 때문에 무조건 연속된 문자열만 제거해야 한다고 착각해서 헤멨는데, 사실 원하는 문자들만 골라서 회문으로 만들 수 있으면 되는 문제였다. ‘a’의 전부 또는 ‘b’의 전부는 항상 회문이기 때문에 답은 항상 0, 1, 2 중 하나가 된다.

Other Solutions

1st

class Solution:
    def removePalindromeSub(self, s):
        return 2 - (s == s[::-1]) - (s == "")

time complexity: 𝑂(𝑛)
space complexity: 𝑂(1)