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

依然递归，对左右子树分别求最大深度，然后取较大的+1

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null)
            return 0;

        int left = maxDepth(root.left);
        int right = maxDepth(root.right);

        return 1 + Math.max(left, right);
    }
}