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Longest Palindromic Substring

medium
Acceptance: 42%
StringsTwo Pointers

For a given string, find and return the longest substring that reads the same forwards and backwards. If multiple substrings share the maximum length, any one of them is acceptable.

Examples

Example 1:
Input:s = "racecar"
Output:"racecar"
Explanation: The entire string is a palindrome.
Example 2:
Input:s = "abcda"
Output:"a"
Explanation: No multi-character palindromic substring exists, so any single character is valid.

Hints

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class Solution {
    public String longestPalindrome(String s) {
        int start = 0, maxLen = 0;
        for (int i = 0; i < s.length(); i++) {
            int len1 = expand(s, i, i);
            int len2 = expand(s, i, i + 1);
            int len = Math.max(len1, len2);
            if (len > maxLen) {
                maxLen = len;
                start = i - (len - 1) / 2;
            }
        }
        return s.substring(start, start + maxLen);
    }

    private int expand(String s, int left, int right) {
        while (left >= 0 && right < s.length() && s.charAt(left) == s.charAt(right)) {
            left--;
            right++;
        }
        return right - left - 1;
    }
}
Time complexityO(n^2)
Space complexityO(1)