125 验证回文串

https://leetcode.cn/leetbook/read/top-interview-questions-easy/xne8id/

https://leetcode.cn/problems/valid-palindrome/description/

cost: 13m24s

class Solution {
public:
    bool isPalindrome(string s) {
        for (int i = 0, j = s.size() - 1; i < j;) {
            char ch1 = s[i];
            char ch2 = s[j];
            if (!this->isAlpha(ch1) && !this->isNum(ch1)) {
                i++;
                continue;
            }
            if (!this->isAlpha(ch2) && !this->isNum(ch2)) {
                j--;
                continue;
            }
            if (!this->isEqualChar(ch1, ch2)) {
                return false;
            }
            i++;
            j--;
        }
        return true;
    }

private:
    bool isAlpha(char ch) {
        if (ch >= 'a' && ch <= 'z') {
            return true;
        }
        if (ch >= 'A' && ch <= 'Z') {
            return true;
        }
        return false;
    }
    bool isNum(char ch) { return ch >= '0' && ch <= '9'; }
    char toLower(char ch) {
        if (ch >= 'A' && ch <= 'Z') {
            return ch - 'A' + 'a';
        }
        return ch;
    }
    bool isEqualChar(char ch1, char ch2) {
        if (ch1 == ch2) {
            return true;
        }
        if (this->toLower(ch1) == this->toLower(ch2)) {
            return true;
        }
        return false;
    }
};

• 时间复杂度：O(n)
• 空间复杂度：O(1)