LeetCode 1332. Remove Palindromic Subsequences

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'.

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这题读起来有点绕，就是说从一个由a和b组成的string里remove一个palindrome subsequence，注意这里的条件既要palindrome又是subsequence（也就是随便从这个string里抽字符出来就行，而不是substring），刚开始觉得有点懵，但是仔细一想其实因为只有a和b，所以最坏情况下一次remove全a和一次remove全b就行了，最好情况下就是本身是palindrome的话就直接全部remove了，所以这题就退化成了简单的isPalindrome。

class Solution {
    public int removePalindromeSub(String s) {
        int start = 0;
        int end = s.length() - 1;
        while (start < end) {
            if (s.charAt(start) != s.charAt(end)) {
                return 2;
            }
            start++;
            end--;
        }
        return 1;
    }
}