Compare Normalized ASCII Phrase Symmetry
Company: Google
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: easy
Interview Round: Other
### Problem
Check whether a string reads the same backward after removing every non-alphanumeric character and treating uppercase letters as lowercase. Alphanumeric means English letters and decimal digits. If filtering leaves no characters, the result is a palindrome.
```cpp
bool isPalindrome(string s);
```
### Examples
```text
Input: s = "No 'x' in Nixon!"
Output: true
```
Its normalized form is `noxinnixon`.
```text
Input: s = "0P"
Output: false
```
Digits are retained; `0p` is not symmetric.
### Constraints
- The length is between 1 and 200000.
- Every character is printable ASCII.
Overview: Check a printable-ASCII phrase for palindrome symmetry while ignoring punctuation and letter case but retaining digits.