算法-字符串-最长回文子串

class Solution {
    
    
    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 = help(s, i, i);//bab
            int len2 = help(s, i, i + 1);//baab
            int len = Math.max(len1, len2);

            if(len >= end -start + 1) {
    
    //新长度大于等于就长度就记录下来
                //重新计算值

                start = i - (len - 1) / 2;
                end = i + (len) / 2;
            }

            
        }
        return s.substring(start, end + 1);
    }



    public int help(String s, int left, int right) {
    
    
        while(0 <= left && right < s.length() && s.charAt(left) == s.charAt(right)) {
    
    
            left--;
            right++;
        }

        return right - left - 1;
    }
}