**需求：**给一个字符串，获取最长回文子串

示例：

输入："abbadc"
输出："bad"

**思路：**中心扩散法
字符串挨个字符遍历过去，左右扩散判断是否是回文数，记录长度和下标

public String longestPalindrome(String s) {
    if (s.length()<=1){
    return s;
	}
	int left,right = 0;
	int len = 1;
	int maxlen = 0;
	int maxStart = 0;
	for (int i = 0; i < s.length(); i++) {
	    left = i - 1;
	    right = i + 1;
	    while (left >= 0 && s.charAt(left) == s.charAt(i)){
	        len++;
	        left--;
	    }
	    while (right <= s.length()-1 && s.charAt(right) == s.charAt(i)){
	        len++;
	        right++;
	    }
	    while (left >= 0 && right <= s.length()-1 && s.charAt(left) == s.charAt(right)){
	        len += 2;
	        left--;
	        right++;
	    }
	    if (len > maxlen) {
	        maxlen = len;
	        maxStart = left + 1;
	    }
	    len = 1;
	}
	return s.substring(maxStart, maxStart + maxlen);
}