Two Pointers

Memorise one skeleton. Every opposite-ends problem is this loop with the if/else body swapped.

Compare characters from both ends while skipping non-alphanumeric characters and ignoring case.

Maintain a “write” position for the next non-zero while scanning with a “read” pointer (or compact with a single slow/fast index).

Swap characters symmetrically from both ends until pointers meet; char[] is mutable, so this is true O(1) extra space.

In a sorted array, the largest squares come from the ends (most negative or most positive); merge into a result array from right to left.

Runnable Java file — no imports, no Maven needed. Save it and run java ValidPalindrome.java.