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.

Approach 01: Expand Around Center

class Solution {
public:
    string longestPalindrome(string s) {
        if (s.empty()) return "";
        int start = 0, end = 0;
        for (int i = 0; i < s.length(); i++) {
            int len1 = expandAroundCenter(s, i, i);
            int len2 = expandAroundCenter(s, i, i + 1);
            int len = max(len1, len2);
            if (len > end - start) {
                start = i - (len - 1) / 2;
                end = i + len / 2;
            }
        }
        return s.substr(start, end - start + 1);
    }
private:
    int expandAroundCenter(string s, int left, int right) {
        while (left >= 0 && right < s.length() && s[left] == s[right]) {
            left--;
            right++;
        }
        return right - left - 1;
    }
};

class Solution:
    def longestPalindrome(self, s: str) -> str:
        if not s: return ""
        start = 0
        end = 0
        def expandAroundCenter(left, right):
            while left >= 0 and right < len(s) and s[left] == s[right]:
                left -= 1
                right += 1
            return right - left - 1
        for i in range(len(s)):
            len1 = expandAroundCenter(i, i)
            len2 = expandAroundCenter(i, i + 1)
            length = max(len1, len2)
            if length > end - start:
                start = i - (length - 1) // 2
                end = i + length // 2
        return s[start:end+1]

class Solution {
    public String longestPalindrome(String s) {
        if (s == null || s.length() < 1) return "";
        int start = 0, end = 0;
        for (int i = 0; i < s.length(); i++) {
            int len1 = expandAroundCenter(s, i, i);
            int len2 = expandAroundCenter(s, i, i + 1);
            int len = Math.max(len1, len2);
            if (len > end - start) {
                start = i - (len - 1) / 2;
                end = i + len / 2;
            }
        }
        return s.substring(start, end + 1);
    }
    private int expandAroundCenter(String s, int left, int right) {
        while (left >= 0 && right < s.length() && s.charAt(left) == s.charAt(right)) {
            left--;
            right++;
        }
        return right - left - 1;
    }
}