LeetCode125 Valid Palindrome

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

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.

分析：

回文判断思路比较直观就是两根指针一前一后比较即可。但本题中只判断字母和数字，其他字符直接忽略，所以要加一些处理。

代码：

class Solution {
public:
    bool isPalindrome(string s) {
        if (s.size() == 0) {
            return true;
        }
        for (int i = 0; i < s.size(); ++i) {
            if (s[i] >= 'A' && s[i] <= 'Z') {
                s[i] += ('a' - 'A');
            }
        }
        int i = 0, j = s.size() - 1;
        while (i <= j) {
            if ( !( (s[i] >= 'a' && s[i] <= 'z') || (s[i] >= '0' && s[i] <= '9')) ) {
                i++;
                continue;
            }
            if (!( (s[j] >= 'a' && s[j] <= 'z') || (s[j] >= '0' && s[j] <= '9') )) {
                j--;
                continue;
            }
            if (s[i] != s[j] ) {
                return false;
            }
            i++;
            j--;
        }
        return true;
    }
};