LeetCode——valid-palindrome

Question

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.

Solution

把不是数字和字母的字符先去掉，然后统计大小写，最后判断是否为回文。

Code

class Solution {
public:
    bool isPalindrome(string s) {
        if (s.empty())
            return true;
        
        string str;
        
        for (char c : s) {
            if (isDigitAlpha(c)) {
				if (c >= 'a' && c <= 'z')
                    c -= 32;
                str += c;
            }
        }
        
        s = str;
        
        return isPalindromeCore(s, 0, s.length() - 1);
    }
    
    bool isDigitAlpha(char c) {
        if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9')) {
            return true;
        }
        return false;
    }
    bool isPalindromeCore(string s, int start, int end) {
        while (start <= end) {
            if (s[start++] != s[end--])
                return false;
        }
        return true;
    }
};