leetcode:Valid Palindrome 合法回文

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.
这题的意思就是判断一串字符串是否是回文，只考虑字母数字的情况。
alphanumeric 是字母数字的意思，我一开始以为是字母，看我写的那个判断函数名就知道了

,结果可想而知，深深的感到英语这厮对我充满了恶意，

class Solution {
public:
bool isPalindrome(string s) {

int tail = s.length() -1 ;

int head = 0;

while (head < tail)
{
while (!isAlpha(s[tail]) && tail>head)
--tail;
while (!isAlpha(s[head]) && head<tail)
++head;

if ( tolower(s[head]) != tolower(s[tail]))
return false;

++head;
--tail;
}
return true;
}
private:
bool isAlpha( char c)
{
if(('A'<=c && c<='Z')||('a'<=c && c<='z')|| ('0'<=c && c<='9'))
return true;
return false;
}
};