【Leetcode】Valid Palindrome

题目：

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.

解题思路：设置首尾两个迭代器，从两头向中间遍历即可。只访问数字和字母（需要注意string里的字幕可能会有大小写区别，然而题目要求忽略大小写，因此比较之前需要将字母全部转换成小写或全部转换成大写）。

代码：

class Solution {
public:
    bool isPalindrome(string s) {
        if(s.empty())return true;
        string::iterator iter_front=s.begin(),iter_end=s.end()-1;
        while(iter_front<iter_end){
            if(!isalnum(*iter_front)){iter_front++;}
            else if(!isalnum(*iter_end)){iter_end--;}
            else if(tolower(*iter_front)==tolower(*iter_end)){
                iter_front++;
                iter_end--;
            }else{
                return false;
            }
        }
        return true;
    }
};