给一个字符串，找到其最长的回文字串

 1 string expendCentral(string, int);
 2 
 3 string longestPalindrome(string s)
 4 {
 5     int len = s.size();
 6     if (len == 0)
 7         return "";
 8     string maxStr = s.substr(0, 1);
 9     for (int i = 0; i < len - 1; i++) {
10         string expendStr = expendCentral(s, i);
11         if (expendStr.size() > maxStr.size())
12             maxStr = expendStr;
13     }
14     return maxStr;
15 }
16 
17 string expendCentral(string s, int d)
18 {
19     int l1 = d;
20     int r1 = d;
21     while (l1 - 1 >= 0 && r1 + 1 < s.size()) {
22         if (s[l1-1] == s[r1+1]) {
23             l1--;
24             r1++;
25         }
26     }
27     int len1 = r1 - l1 + 1;
28     int len2 = 0;
29     int l2 = d;
30     int r2 = d + 1;
31     if (s[d] == s[d+1]) {
32         while (l2 - 1 >= 0 && r2 + 1 < s.size()) {
33             if (s[l2-1] == s[r2+1]) {
34                 l2--;
35                 r2++;
36             }
37         }
38         len2 = r2 - l2 + 1;
39     }
40     if (len1 > len2)
41         return s.substr(l1, r1-l1+1);
42     else
43         return s.substr(l2, r2-l2+1);
44 }