Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

 这题是看答案写出来的，解题思路就是头尾匹配再前后数组下标向中间移位1，continue在这的作用有点费解，没有的话会报错，待研究。。。

 1 class Solution {
 2 public:
 3     bool isValid(char c) {
 4     return (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9');
 5 }
 6 bool isPalindrome(string s) {
 7     int start = 0, end = s.size() - 1;
 8     while (start < end) {
 9         if (!isValid(s[start])) { start ++; continue; }
10         if (!isValid(s[end])) { end --; continue; }
11         if (s[start] != s[end] && abs(s[start] - s[end]) != 32) return false;
12         start ++;
13         end --;
14     }
15     return true;
16 }
17 };