125. Valid Palindrome

Easy

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:

Approach 1:

class Solution {
    public boolean isPalindrome(String s) {
        String str = s.toLowerCase().replaceAll("[^a-z0-9]","");
        int i = 0;
        int j = str.length()-1;
        while(j>=i){
            if(str.charAt(i) != str.charAt(j)){
                return false;
            }
            
            i++;
            j--;
        }
        return true;
        
    }
    
}

Approach 2:

 class Solution {
     public boolean isPalindrome(String s) {
        
         String user_string = "";
         String reverse_string = "";
        
         for(int i=0;i<s.length();i++)
         {
             if(s.charAt(i)>='a' && s.charAt(i) <='z' || s.charAt(i)>='A' && s.charAt(i) <='Z'  || s.charAt(i)>= '0' && s.charAt(i) <='9')
             {
                 user_string = user_string + Character.toLowerCase(s.charAt(i));
             }
            
         }
        
         for(int j=user_string.length()-1;j>=0;j--){
             reverse_string = reverse_string + user_string.charAt(j);
         }
        
         if(user_string.equals(reverse_string))
             return true;
         return false;
     }
    
 }