[Leetcode]Valid Palindrome

思路：头尾双指针向中间靠拢

class Solution {public:    bool isPalindrome(string s) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        if(s.empty()) return true;        for(string::size_type i = 0; i < s.size(); ++i)            s[i] = tolower(s[i]);        int l = 0, r = s.size() - 1;        while(l < r)        {            while(l < r && !isalnum(s[l])) l++;            while(l < r && !isalnum(s[r])) r--;            if(s[l] != s[r]) return false;            l++, r--;        }        return true;    }};