125. Valid Palindrome(Solution || Leetcode easy || Java)

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Given a string s, return true if it is a palindrome, or false otherwise.

Example 1:

Input: s = "A man, a plan, a canal: Panama"
Output: true
Explanation: "amanaplanacanalpanama" is a palindrome.

Example 2:

Input: s = "race a car"
Output: false
Explanation: "raceacar" is not a palindrome.

Example 3:

Input: s = " "
Output: true
Explanation: s is an empty string "" after removing non-alphanumeric characters.
Since an empty string reads the same forward and backward, it is a palindrome.

Constraints:

• 1 <= s.length <= 2 * 105
• s consists only of printable ASCII characters.

SOLUTION:

class Solution {
    public boolean isPalindrome(String s) {
//Convert all the letters to lowercase
        s = s.toLowerCase();

//Create stringbuilder to consume less space while modifying string
        StringBuilder temp = new StringBuilder();
  
//add all the alphanumeric to temp      
        for(int i = 0; i < s.length(); i++){
            if((s.charAt(i) >= '0' && s.charAt(i) <= '9') || 
            (s.charAt(i) >= 'a' &&   s.charAt(i) <= 'z')){
                temp.append(s.charAt(i));
            }
        }
        
        int j = temp.length() - 1;
  
//run loop to see if it is a palindrome or not      
        for(int i = 0; i < (temp.length()/2); i++,j--){
            if(temp.charAt(i) != temp.charAt(j)){
                return false;
            }
            
        }
        
        return true;
    }
}

Runtime: 5 ms, faster than 88.51% of Java online submissions for Valid Palindrome.

Memory Usage: 42.2 MB, less than 96.46% of Java online submissions for Valid Palindrome.

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