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# 01/31/2022 125

Method1: create a stringBuilder object, iterate the String, only append the alphanumeric character and also convert it to lower case letter. Then return the value of comparing the string with the reversed string. 

Time O(n)

Space O(n)

```
class Solution { 
    public boolean isPalindrome(String s) { 
    StringBuilder st = new StringBuilder(); 
    for(char ch: s.toCharArray()){ 
    if(Character.isLetterOrDigit(ch)){ 
    st.append(Character.toLowerCase(ch)); } }
    String filtered = st.toString();
    String reversed = st.reverse().toString();
    
    return filtered.equals(reversed);
}
```

}

Method 2:

Use two pointers. One is from the left and another is from the right. Skip the non-alphanumeric characters and compare them iteratively. 

Time O(N)

Space O(1)

(use while loop and remember the conditions check: left < right && Character.isLetterOrDigit())&#x20;

```
class Solution { 
    public boolean isPalindrome(String s) { 
    s = s.toLowerCase();// remember
    int left = 0;
    int right = s.length() - 1;
    while(left < right){
        while (left < right && !Character.isLetterOrDigit(s.charAt(left))) left++;// need to remebeer
        while (left < right && !Character.isLetterOrDigit(s.charAt(right))) right--;
        if(s.charAt(left) != s.charAt(right)){
            return false;
        }
        left++;
        right--;         
    }
    return true;
}
```

}