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;
}
};