Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Notice

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.

Have you met this question in a real interview?

 

 

Example

"A man, a plan, a canal: Panama" is a palindrome.

"race a car" is not a palindrome.

O(n) time without extra memory.

 

LeetCode上的原题，请参见我之前的博客。

 

解法一：

class Solution {public:    /**     * @param s A string     * @return Whether the string is a valid palindrome     */    bool isPalindrome(string& s) {        int left = 0, right = s.size() - 1;        while (left < right) {            if (!isAlNum(s[left])) ++left;            else if (!isAlNum(s[right])) --right;            else if ((s[left] + 32 - 'a') % 32 != (s[right] + 32 - 'a') % 32) return false;            else {                ++left; --right;            }        }        return true;    }    bool isAlNum(char c) {        if (c >= 'a' && c <= 'z') return true;        if (c >= 'A' && c <= 'Z') return true;        if (c >= '0' && c <= '9') return true;        return false;    }};

 

解法二：

class Solution {public:    /**     * @param s A string     * @return Whether the string is a valid palindrome     */    bool isPalindrome(string& s) {        int left = 0, right = s.size() - 1;        while (left < right) {            if (!isalnum(s[left])) ++left;            else if (!isalnum(s[right])) --right;            else if ((s[left] + 32 - 'a') % 32 != (s[right] + 32 - 'a') % 32) return false;            else {                ++left; --right;            }        }        return true;    }};