Leetcode 89. Gray Code

Generate any sequence of 2^n distinct integers in [0, 2^n-1] starting at 0 such that consecutive numbers (and the first and last) differ by exactly one bit—equivalently, produce a Hamiltonian cycle/traversal of the n-dimensional hypercube. Constraint: 1 ≤ n ≤ 16 (output length 2^n).