You get a nested list nested. Each item in it is a whole number or another list. Those inner lists hold numbers and lists too.
Return all the numbers in one flat list. Keep them in the order they appear.
The starter code shows how Java holds the items.
Example 1:
Input: nested = [7,[-2,[5]],[],4] Output: [7,-2,5,4] Explanation: The 7 comes first. The list [-2,[5]] gives -2, then 5. The empty list [] gives nothing. The 4 comes last.
Example 2:
Input: nested = [[[]],[9,9],[]] Output: [9,9] Explanation: Only the list [9,9] holds numbers. Both 9s stay.
Example 3:
Input: nested = [[[[12]]]] Output: [12] Explanation: The 12 sits inside four lists.
Constraints:
- 0 to 10,000 numbers in all.
- At most 10,000 lists, the outer one included.
- Lists nest at most 50 deep, the outer one included.
- Each number is a whole number from -100,000 to 100,000.
Your solution should run in O(n) time, where n counts the numbers and the lists.
Go through the items from left to right. A number goes straight to the end of the answer.
An item that is a list is the same problem, only smaller. Flatten it with the same function, adding its numbers to the same answer. Then go on with the next item.
Each number and each list is visited once, so the time is O(n).
Besides the answer, the recursion uses one call per level of nesting: O(d) extra space for
d levels.
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