Restore by reversing twice
Updated
Two reverses are identity: reverse(reverse(a)) = a. Using a stack twice returns the original order. One reverse is the transformation. Two is a round trip (and a way to copy through a helper without flipping).
What to focus on
Copy-via-stack once reverses. Copy-via-stack twice (or stack then stack) restores. One-item is identity either way.
Worked examples
round trip
a = [4, 1, 9], reverse, reverse- [9, 1, 4] then [4, 1, 9]
Result: [4, 1, 9]
one reverse
a = [4, 1, 9], reverse once- [9, 1, 4]
Result: not restored
one item
a = [7], reverse twice- still [7]
Result: [7]
Try it
Write reverseTwice(a). a is an array of integers. Reverse with a stack twice and return the original. One reverse is not enough. Reverse [4, 1, 9] twice → [4, 1, 9]. Reverse once → [9, 1, 4]. [7] stays [7].
a is a non-empty list of integers.