PalindromeSubstring.java

class Solution {
    
    /**
     * validate if string is palindrome
     */
  
    
    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); // odd  case
            int len2 = expandAroundCenter(s, i, i + 1); // even case
            
            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; 
    }
}