You are given an array of distinct integers. Your task: return all possible permutations of the array.
Example: [1,2,3] has 6 permutations: [1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], [3,2,1].
This is a classic backtracking problem. You build permutations step by step, trying each element in each position.
Try it yourself first.