125. 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.

思路：

本题思路简单，判断字符串是否为回文，属于面试经典题目，步骤如下：

1. 将字符串标点去除；

2. 将字符串中空格去除；

3. 将字符串小写字母转换为大写字母，统一表示

4. 建立两个指针指向字符串首尾，依次比较

代码：

class Solution {public:    bool isPalindrome(string s) {        if(s.size()==0)            return true;                    string::iterator itr = s.begin();        while(itr!=s.end())        {            if(ispunct(*itr))            {                s.erase(itr);                continue;            }            else if(*itr==' ')            {                s.erase(itr);                continue;            }            itr++;        }        for(int i =0;i<s.size();i++)        {            if(s[i]>='a')                s[i] -= 32;        }                    string::iterator begin = s.begin();                string::iterator end = s.end();        end--;        for(;begin<end;begin++,end--)        {            if(*begin!=*end)            return false;        }                return true;         }};