如果记得 3.3binary search tree里面关于height的code，就可以瞬间写出bug free的solution了。
以下是3.3binary search tree里面关于height的code：
/**
* Returns the height of the BST (for debugging).
*
* @return the height of the BST (a 1-node tree has height 0)
*/
public int height() {
return height(root);
}
private int height(Node x) {
if (x == null) return -1;
return 1 + Math.max(height(x.left), height(x.right));
}
//以下是104题的解法：
/**
* Definition for a binary tree node.
* 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;
return 1 + Math.max(root.left, root.right);
}
}