leetcode 55: Valid Palindrome

Valid PalindromeJan 13

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.

 

 

class Solution {
public:
    bool isPalindrome(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if( s.size() == 0 ) return true;
        
        int sz = s.size();
        
        int i = 0, j=sz-1;
        
        while(i<j) {
            while( !isValid(s[i]) && i<j) i++;
            while( !isValid(s[j]) && i<j) j--;
            if(i>=j) break;
            if( s[i] != s[j] ) return false;
            else {
                ++i;
                --j;
            }
        }
        return true;
        
    }
    
private:
    bool isValid( char & c) {
        
        c = lowerCase(c);
        if( c>='0' && c<='9' ) return true;
        if( c>='a' && c<='z' ) return true;
        if( c>='A' && c<='Z' ) return true;
        return false;
    }
    
    char lowerCase( char c) {
        if( c>='A' && c<='Z') return c + 'a' - 'A';
        else return c;
    }
    
};