Time complexity of a recursive function

#1

how does the last level has 2^n-1 nodes.

#2

For a complete binary tree
The 1st level has 1 node (which is the root node)
2nd level has 2 node
3rd level – 4 nodes
4th level – 8 nodes ie. 2^3

So nth level – 2^(n-1) nodes

You can draw the tree and verify

#3

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