The height of this tree is ______. (write number only) code example

Example: The height of this tree is ______. (write number only

Input: Given binary tree, 
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
      5
     / \
    3   6
   / \
  2   4
 /
1
Output: return its depth = 4