題目為：

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.

階梯思路：對於這種題目最簡單的方法就是遞迴操作了

程式碼為：

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null)
            return 0;
       else if(root.left == null && root.right == null)
            return 1;
       else
            {
                int leftDepth= maxDepth(root.left);
       			int rightDepth = maxDepth(root.right);
                if(leftDepth > rightDepth)
                    return leftDepth +1;
                else
                    return rightDepth +1;
             }
       
    }
}