leetcode 125

125. Valid Palindrome

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.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

判断字符串是否为回文字符串，只需考虑字符串中的数字和字母。

思路：字符串首尾各一个指针，进行比较；

　　　指针移动规则：若相等则首指针向前移动一位，尾指针向后移动以为；若出现非法字符则首尾指针相应的移动一位。

代码如下：

 class Solution {
 public:
     bool isPalindrome(string s) {
         int n = s.size();
         int st = ;
         if(n == )
         {
             return true;
         }
         n--;
         while(st < n)
         {
             if(isValid(s[st]) && isValid(s[n]))
             {
                 if(s[st] <=  && s[st] >= )
                 {
                     if(s[n] >  || s[n] < )
                     {

                         return false;
                     }
                     else if(s[st] != s[n])
                     {
                         return false;
                     }
                     else
                     {
                         st++;
                         n--;
                         continue;
                     }
                 }
                 if(s[st] == s[n] || s[st]-s[n] ==  || s[n]-s[st] == )
                 {
                     st++;
                     n--;
                     continue;
                 }
                 else
                 {
                     return false;
                 }
             }
             else if(isValid(s[st]))
             {
                 n--;
             }
             else if(isValid(s[n]))
             {
                 st++;
             }
             else
             {
                 n--;
                 st++;
             }
         }
         return true;
     }
     bool isValid(char s)
     {
         if(s <  || s > )
         {
             return false;
         }
         else if(s >  && s < )
         {
             return false;
         }
         else if(s >  && s < )
         {
             return false;
         }
         return true;
     }
 };