Longest Palindromic Substring

longestPalindrome.java

/*
Given a string S, find the longest palindromic substring in S. 
You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.

Algorithm:
1. It is a pretty straight forward brute force algorithm, and just move 
the center of the parlindrome forward and keep track of the max palindrome
2. There are 2 different situation: 
  a. the center can be even
  b. the center can be odd
3. The big O time complexity will be n^2, and space complexity will be constant.
*/
public class Solution {
    public String longestPalindrome(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        String evenMax = longestPalindromeHelper(s, 0, 1);
        String oddMax = longestPalindromeHelper2(s, 1);
        return evenMax.length() > oddMax.length() ? evenMax : oddMax;
    }
    
    public String longestPalindromeHelper(String str, int center1, int center2) {
        if(str.length() < 2) return str;
        
        int moveLeft;
        int moveRight;
        String curMax = "";
        while(center2 < str.length()) {
            moveLeft = center1;
            moveRight = center2;
            while(moveLeft >= 0 && moveRight < str.length() &&
                isPalindrome(str, moveLeft, moveRight)) {
                if((moveRight - moveLeft + 1) > curMax.length()) {
                    curMax = str.substring(moveLeft, moveRight+1);
                }
                moveLeft--;
                moveRight++;
            }
            center1++;
            center2++;
        }
        return curMax;
    }
    
    public String longestPalindromeHelper2(String str, int center) {
        if(str.length() < 3) return str;
        int moveLeft;
        int moveRight;
        String curMax = "";
        while(center < str.length()) {
            moveLeft = center - 1;
            moveRight = center + 1;
            while(moveLeft >= 0 && moveRight < str.length() &&
                    isPalindrome(str, moveLeft, moveRight)) {
                if ((moveRight - moveLeft + 1) > curMax.length()) {
                    curMax = str.substring(moveLeft, moveRight+1);
                }
                moveLeft--;
                moveRight++;
            }
            center++;
        }
        return curMax;
    }
    
    public boolean isPalindrome(String str, int left, int right) {
        return str.charAt(left) == str.charAt(right);
    }
}