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.

思路：有效回文。需要一个指向头部的变量和指向尾部的变量，从字符串的两边来遍历，遇到除字母和数字以外的字符则跳过，如果遍历完整个字符串后前后字母都能匹配上，则是回稳返回true.注意：空串也是回文。

        需要注意的几点：1、对大小写不敏感，需要把所有字母小写都转换成大写，或者大写都转换成小写，用toupper()函数。

                               2、遇到非字母、数字要跳过，用到isalnum()函数

#include <iostream>
#include "ctype.h"
using namespace std;

bool isPalindrome(string s)
{
    int first,last;
    first = 0;
    last = s.size() - 1;
    while (first <= last)
    {
        while (!(isalnum(s[first])) && first < last)
        {
            first++;
        }
        while (!(isalnum(s[last])) && first < last)
        {
            last--;
        }
        if (toupper(s[first]) != toupper(s[last]))
        {
            return false;
        }
        first++;
        last--;
    }
    return true;
}

void main()
{
    string s = "A man, a plan, a canal: Panama";
    bool m = isPalindrome(s);
    cout << m << endl;
    system("pause");
}