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.

判断是否是回文，比较常规和简单的问题。其中需要跳过不是字母和数字的内容。

空字符串也属于回文。

两个指针分别从头和尾两端向中间扫描，跳过无效字符，判断是否相等即可。

代码：

class Solution {
public:
    bool isPalindrome(string s) {
        int left=0,right=s.size()-1;	//两头向中间扫描
		while(left<right)
		{
			while(left<s.size()&&!isalnum(s[left]))	//跳过无效字符
				left++;
			while(right>=0&&!isalnum(s[right]))
				right--;
			if(left>=right)	//结束
				return true;
			if(s[left]>='A'&&s[left]<='Z')	//转为小写
				s[left]+='a'-'A';
			if(s[right]>='A'&&s[right]<='Z')
				s[right]+='a'-'A';
			if(s[left]!=s[right])	//不等
				return false;
			else	//相等
			{
				left++;
				right--;
			}
		}
		return true;
    }
};