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.

思路：从两头开始往中间靠拢，需要进行比对。

#include <iostream>
#include <vector>
#include <string>
using namespace std;
/*
判断一个字符串是否为回文字符串 这里只比较字符，大写字符和小写字符
看成是一样的 
*/ 

bool helper(string& str,int begin,int end)
{
	if(begin == end)
		return true;
	if(begin > end)
		return false;
	if(isalpha(str[begin]) && isalpha(str[end]))
	{
		if(toupper(str[begin]) == toupper(str[end]))
			return helper(str,begin+1,end-1);
		else
			return false;
	}
	else if(!isalpha(str[begin]) && isalpha(str[end]))
		return helper(str,begin+1,end);
	else if(isalpha(str[begin]) && !isalpha(str[end]))
		return helper(str,begin,end-1);
	else
		return helper(str,begin+1,end-1);
}

bool isVaildPalindrome(string& str)
{
	if(str.length()==0)
		return 0;
	return helper(str,0,str.length()-1);
}

int main()
{
	string str("A man, a plan, a canal: Panama");
	cout<<isVaildPalindrome(str);
	return 0;
}