Valid Palindrome

描述
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.

题意：给定一个字符串，判断是否含有回文，仅考虑字母和数字。空字符串也是有效的回文。

分析：本文仅考虑字母和数字，并且忽略大小写。
通过将之前和之后的进行对比，进行字符判断。

class Solution {public:  bool isPalindrome(string s) {    int n = s.size();    int i=0, j=n-1;    while(i<j){      if( !isalpha(s[i]) ) {        ++i;        continue;//结束单次循环。      }      if( !isalpha(s[j]) ) {        --j;        continue;      }      if(s[i] != s[j]) return false;      ++i,--j;    }    return true;  }  //判断是否是字母数字，如果是大写字母则将其转化为小写字母  bool isalpha(char &c){    if((c>='A'&&c<='Z')){      c = c-'A'+'a';      return true;    }    return (c>='a'&&c<='z') || (c>='0'&&c<='9');  }};