Remove Palindromic Subsequences (E)

题目

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.

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: String is already palindrome

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".

Example 4:

Input: s = ""
Output: 0

Constraints:

• 0 <= s.length <= 1000
• s only consists of letters 'a' and 'b'

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题意

给定一个只包含'a'和'b'的字符串s，每次可以从s中删去一个回文子序列，问多少次操作后可以删光s。

思路

注意这里的回文子序列不一定需要其中字符在s中是相连的。考虑三种情况：1. s为空，直接返回0；2. s本身是回文串，则可以一次删光；3. s不是回文，则至少需要2次删光，又因为s只包含两种字符，可以第一次删除全'a'子序列，第二次删除全'b'子序列，保证能2次删光s。

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代码实现

Java

class Solution {
    public int removePalindromeSub(String s) {
        return s.isEmpty() ? 0 : isPalindrome(s) ? 1 : 2;
    }

    private boolean isPalindrome(String s) {
        int i = 0, j = s.length() - 1;
        while (i <= j) {
            if (s.charAt(i++) != s.charAt(j--)) return false;
        }
        return true;
    }
}