给你一棵 完全二叉树 的根节点 root ，求出该树的节点个数。

递归遍历1

public static int countNodes(TreeNode root){return preorder(root);}//计算树的节点个数public static int preorder(TreeNode root){int count=0;if(root==null) return 0;int left=preorder(root.left);int right=preorder(root.right);count=left+right+1;  //树的节点个数=左子树节点个数+右子树节点个数+1(根节点)return count;}

递归遍历2（精简版）

 public int countNodes(TreeNode root) {if(root == null) {return 0;}return countNodes(root.left) + countNodes(root.right) + 1;}