LeetCode125. 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.

题目分析：

很简单，忽略掉非字母的，然后前后循环，然后时间复杂度在O(n/2)。

代码：

class Solution {
public:
	bool isPalindrome(string s) {
		int start = 0, end = s.length() - 1;
		while (start<end) {
			if (!isalnum(s[start])) start++;
			else if (!isalnum(s[end])) end--;
			else {
				if (tolower(s[start++]) != tolower(s[end--])) return false;
			}
		}
		return true;
	}
};