90. Subsets II

subsets([1,2,3,4]) = []
                     // push(1)
                     [1, subsets([2,3,4])] // if push N times in subsets([2,3,4]), the pop times is also N, so vec is also [1] after backtrack.
                     // pop(), push(2)
                     [2, subsets([3,4])]
                     // pop(), push(3)
                     [3, subsets([4])]
                     // pop(), push(4)
                     [4, subsets([])]
                     // pop()