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.

 思路：根节点的最大深度 = 左右子节点中最大深度+1

var maxDepth = function(root) {    if(root === null) return 0;        var ld = maxDepth(root.left);    var rd = maxDepth(root.right);    return ld > rd ? ++ld : ++rd;　　//写法等同于return Math.max(ld,rd)+1;    };