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 7return its depth = 3.
与相等二叉树和对称二叉树的题目一致，用的递归的方法，且先处理无左右节点的根节点为空的情况。
class Solution {
public int maxDepth(TreeNode root) {
if(root==null){return 0;}//如果空二叉树（根节点为空）则返回0；
else{
return (1 + Math.max(maxDepth(root.left),maxDepth(root.right)));//如果根节点不为空，则返回1+左右树的最大深度。
}
}
}