Given a binary tree, write a program to find the maximum depth of the binary tree.

Problem Note

• The maximum depth is the number of nodes along the longest path from the root node to leaf node.
• A leaf is a node with no child nodes.

Example

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