二叉树的最大深度

题目：

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 [25,20,30,28,35]

    25
   /  \
  20  30
     /  \
    28  35 

//函数封装:
BinarySearchTree.prototype.mxaDepthNode = function(node){			
  if(node == null){
	return 0;	
  }
  var leftMaxDepth = this.mxaDepthNode(node.left);
  var rightMaxDepth = this.mxaDepthNode(node.right);
  return Math.max(leftMaxDepth,rightMaxDepth)+1;
}