Longest Palindromic Substring

Description

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.

Example

Example 1:

Input:"abcdzdcab"
Output:"cdzdc"

Example 2:

Input:"aba"
Output:"aba"

Challenge

O(n2) time is acceptable. Can you do it in O(n) time.

思路：基于中心点枚举的算法，时间复杂度 O(n^2)

public class Solution {
    /**
     * @param s: input string
     * @return: the longest palindromic substring
     */
    public String longestPalindrome(String s) {
        if (s == null || s.length() == 0) {
            return "";
        }
        
        int start = 0, len = 0, longest = 0;
        for (int i = 0; i < s.length(); i++) {
            len = findLongestPalindromeFrom(s, i, i);
            if (len > longest) {
                longest = len;
                start = i - len / 2;
            }
            
            len = findLongestPalindromeFrom(s, i, i + 1);
            if (len > longest) {
                longest = len;
                start = i - len / 2 + 1;
            }
        }
        
        return s.substring(start, start + longest);
    }
    
    private int findLongestPalindromeFrom(String s, int left, int right) {
        int len = 0;
        while (left >= 0 && right < s.length()) {
            if (s.charAt(left) != s.charAt(right)) {
                break;
            }
            len += left == right ? 1 : 2;
            left--;
            right++;
        }
        
        return len;
    }
}
}