[LC] 5. Longest Palindromic Substring

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example 1:

Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.

Example 2:

Input: "cbbd"
Output: "bb"

Solution 1:
Time: O(N^2)

class Solution:
    def longestPalindrome(self, s: str) -> str:
        if len(s) <= 1:
            return s
        self.res = ''
        for i, char in enumerate(s):
            self.helper(i, i, s) # odd
            self.helper(i, i + 1, s) # even
        return self.res

    def helper(self, start, end, s, even=False):
        while start >= 0 and end < len(s):
            if s[start] == s[end]:
                if end - start >= len(self.res):
                    self.res = s[start: end + 1]
                start -= 1
                end += 1
            else:
                return

class Solution {
    int start = 0;
    int maxLen = 0;
    public String longestPalindrome(String s) {
        int len = s.length();
        for (int i = 0; i < len; i++) {
            helper(i, i, s);
            helper(i, i + 1, s);
        }
        return s.substring(start, start + maxLen);
    }

    private void helper(int low, int high, String s) {
        while (low >= 0 && high < s.length() && s.charAt(low) == s.charAt(high)) {
            low -= 1;
            high += 1;
        }
        if (high - low - 1 > maxLen) {
            maxLen = high - low - 1;
            start = low + 1;
        }
    }
}

Solution 2:

Time: O(N^2)

class Solution {
    public String longestPalindrome(String s) {
        if (s == null || s.length() <= 1) {
            return s;
        }
        boolean[][] isPalin = new boolean[s.length()][s.length()];
        int max = 0;
        String res = "";
        for (int i = 1; i < s.length(); i++) {
            // i == j for case of single char
            for (int j = 0; j <= i; j++) {
                if (s.charAt(i) == s.charAt(j) && (i - j <= 2 || isPalin[i - 1][j + 1])) {
                    isPalin[i][j] = true;
                    if (i - j + 1> max) {
                        max = i - j + 1;
                        // j is smaller than i
                        res = s.substring(j, i + 1);
                    }
                }
            }
        }
        return res;
    }
}