You are given an array that may contain nested arrays at any depth.

// Examples of input arrays
[1, 2, 3]
[1, [2, 3]]
[[1, 2], [3, 4]]
[1, [2, [3, [4, [5]]]]]

Your task: Implement flatten(arr) which returns a new array with all nested elements flattened into a single level. The original array should not be modified.

Example

flatten([1, 2, 3]); // [1, 2, 3]
flatten([1, [2, 3]]); // [1, 2, 3]
flatten([[1, 2], [3, 4]]); // [1, 2, 3, 4]
flatten([1, [2, [3, [4, [5]]]]]); // [1, 2, 3, 4, 5]