Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its depth = 3.

Answer:

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def dfs(self,node,level):
        if node==None:
            return level-1
        left=node.left
        right=node.right
        
        return max(self.dfs(left,level+1),self.dfs(right,level+1))
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        return self.dfs(root,1)