Valid Palindrome
easyTwo Pointers+10 XPTime limit 2s
A sentence is a palindrome if, after removing everything except letters and digits and ignoring upper/lower case, it reads the same forwards and backwards. Example: A man, a plan, a canal: Panama → amanaplanacanalpanama → palindrome.
Input: one line s (printable ASCII; it does not start or end with a space).
Output: true or false.
Constraints: 1 ≤ length of s ≤ 2·10⁵.
Challenge: use two pointers from both ends, skipping non-alphanumeric characters — no need to build a cleaned copy.
Sample input 1
A man, a plan, a canal: Panama
Sample output 1
true
Sample input 2
race a car
Sample output 2
false
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Read input from standard input and print the answer to standard output. Your code runs in a secure sandbox against the samples (Run) and hidden tests (Submit).