Find inorder successor in binary search tree

FindInorderSuccessor_Analysis_Study.cs

/*
  Assumptions:
    all keys in the tree are nonnegative
    strictly less than (no equal to)
    if no smaller key return -1
    
      2
     / \   num = 3 -> 2
    1   5  num = 7 -> 5
           num = 2 -> 1           
           
      2   num = 1 -> -1         
      
   Overview:
      recursive approach
      base case: comparing against a node that has no children
                 if key is smaller than num
                    return num
                 else
                    return -1
                    
      standard binary search tree traversal for nodes with children
      
      worst case: O(N) expected case: O(logN)
      size complexity: recrusive so depth of the recursive tree <- potentially improve with iterative
   */