Let me trace through an example. Suppose the input is [2, 3, 4, 1]. This means: friend 1 received from friend 2, friend 2 received from friend 3, friend 3 received from friend 4, friend 4 received from friend 1.
Now reverse it: friend 2 gave to friend 1, friend 3 gave to friend 2, friend 4 gave to friend 3, friend 1 gave to friend 4. So the answer is [4, 1, 2, 3]. To verify: friend 1 gave to friend 4 (check: friend 4 received from friend 1 ✓).
This trace helps you understand the inverse relationship and catch any off-by-one errors in your indexing.