125 Valid Palindrome

public class Solution {
    public boolean isPalindrome(String s) {
        if(s==null)
        	return false;
        s=s.toLowerCase();
        for(int i =0,j=s.length()-1;i<=j;i++,j--){
        	while(!isAlpha(s.charAt(i))&&!isNum(s.charAt(i))){
        		i++;
        		if(i>j)
        			break;
        	}

        	while(!isAlpha(s.charAt(j))&&!isNum(s.charAt(j))){
        		j--;
        		if(i>j)
        			break;
        	}

        	if(i<=j){
        		if(s.charAt(i)!=s.charAt(j)){
            		return false;
            	}
        	}

        }
        return true;
    }
public  boolean isAlpha(char a){
    if((a >= 'a' && a <= 'z') || (a >= 'A' && a <= 'Z')){
        return true;
    }else{
        return false;
    }
}

public  boolean isNum(char a){
    if(a >= '0' && a <= '9'){
        return true;
    }else{
        return false;
    }
}
}