5. Longest Palindromic Substring

Given a string s, return the longest 

palindromic

 

substring

 in s.

Example 1:

Input: s = "babad"
Output: "bab"
Explanation: "aba" is also a valid answer.

Example 2:

Input: s = "cbbd"
Output: "bb"

Constraints:

• 1 <= s.length <= 1000
• s consist of only digits and English letters.

class Solution {
    public String longestPalindrome(String s) {
        int[] result = new int[2]; // To hold the start and maxLen. 这里不用全局变量了。
        
        for (int i = 0; i < s.length(); i++) {
            // Find the longer palindrome for both odd and even lengths
            int[] oddPalindrome = helper(i, i, s); // case like aba
            int[] evenPalindrome = helper(i, i + 1, s); //case like abba

            if (oddPalindrome[1] > result[1]) { //比maxLen大才更新
                result = oddPalindrome;
            }
            if (evenPalindrome[1] > result[1]) {
                result = evenPalindrome;
            }
        }
        
        return s.substring(result[0], result[0] + result[1]);
    }

    private int[] helper(int left, int right, String s) {
        while (left >= 0 && right < s.length() && s.charAt(left) == s.charAt(right)) {
            left--;
            right++;
        }
        // Return the start index and length of the palindrome
        return new int[] { left + 1, right - left - 1 };
    }
}