day21.java

//ques1:5. Longest Palindromic Substring
//link:https://leetcode.com/problems/longest-palindromic-substring/description/


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

//TC:O(n^2)
//SCO(1)