LeetCode Valid Palindrome

class Solution {
public:
    bool isPalindrome(string s) {
        int len = s.length();
        //if (len < 1) return true;
        int p = -1, q = len;
        while (true) {
            char a, b;
            while (++p < len && !(a = check2lower(s[p])) );
            while (--q > -1 && !(b = check2lower(s[q])) );
            if (p >= q) return true;
            if (a != b) {
                return false;
            }
        }
        return true;
    }
    
    char check2lower(char ch) {
        if (ch <= '9' && ch >= '0') return ch;
        if (ch <= 'z' && ch >= 'a') return ch;
        if (ch <= 'Z' && ch >= 'A') return ch + 'a' - 'A';
        return '';
    }
};

跟快排的结构有点类似

第二轮：

class Solution {
public:
    bool isChar(char ch) {
        return ch >= 'A' && ch <= 'Z' || ch >= 'a' && ch <= 'z' || ch >= '0' && ch <= '9';
    }
    
    bool cmpChar(char a, char b) {
        if (a >= 'a') {
            a-=32;
        }
        if (b >= 'a') {
            b-=32;
        }
        return a == b;
    }

    bool isPalindrome(string s) {
        int len = s.size();
        int p = 0;
        int q = len-1;
        while (p<q) {
            while(!isChar(s[p]) && p < q) p++;
            while(!isChar(s[q]) && p < q) q--;
            if (p >= q) break;
            if (cmpChar(s[p], s[q])) {
                p++, q--;
            } else {
                return false;
            }
        }
        return true;
    }
};

　　

第三轮：

class Solution {
public:
    bool isPalindrome(string s) {
        int len = s.size();
        int p = 0;
        int q = len - 1;
        while (p < q) {
            while (p < len && !isAlphanumeric(s[p])) {
                p++;
            }
            while (q > 0 && !isAlphanumeric(s[q])) {
                q--;
            }
            if (p >= q) {
                return true;
            }
            if (toLowerCase(s[p]) != toLowerCase(s[q])) {
                return false;
            }
            p++, q--;
        }
        return true;
    }
    char toLowerCase(char ch) {
        if (ch >= 'a') {
            ch-= 'a' - 'A';
        }
        return ch;
    }
    bool isAlphanumeric(char ch) {
        if (ch >= 'a' && ch <= 'z' 
            || ch >= 'A' && ch <= 'Z'
            || ch >= '0' && ch <= '9') {
            return true;
        }
        return false;
    }
};