Many Would be confused why the recursion depth is not 18 when we have 19 nodes in the recursion call stack, the reason for this is hidden in the definition of recursion depth.
Definition of Recursion Depth : The maximum depth of recursion refers to the number of levels of activation of a procedure which exist during the deepest call of the procedure.
As we can clearly see that recursion depth is Number of levels not the Height of tree so, answer would be 19 not 18