【leetcode刷题笔记】Valid Palindrome

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.

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题解：Using two pointers head and tail, head keeps moving forward till it points to an alpha or number; tail moves backward till it points to an alpha or number, then judge if what head points to equals what tail points, if it doesn't equal, return false; Else cotinue moving head and tail to compare characters bewteen them.

We also need a function isValid(char c) to judge if char c is a number or alpha.

The code is below:

public class Solution {
    private boolean isValid(char c){
        if(c >= 'a' && c <= 'z')
            return true;
        if(c >= '0' && c <='9')
            return true;
        
        return false;
    }
    
    public boolean isPalindrome(String s) {
        s = s.toLowerCase();
        
        if(s == null || s.length() <= 1)
            return true;
        
        int head = 0;
        int tail = s.length() - 1;
        
        while(head < tail){
            while(head < tail && !isValid(s.charAt(head)))
                head++;
            while(tail > head && !isValid(s.charAt(tail)))
                tail--;
            if(s.charAt(head) != s.charAt(tail))
                return false;
            else {
                head++;
                tail--;
            }
            
        }
        
        return true;
    }
}