Count Complete Tree Nodes

Given a complete binary tree, count the number of nodes.

Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h.

思路：

/** * 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 countNodes(TreeNode root) {        if(root==null){            return 0;        }        TreeNode p=root;        int rh=0;        while(p!=null){            rh++;            p=p.right;        }        p=root;        int lh=0;        while(p!=null){            lh++;            p=p.left;        }        if(lh==rh){//左右高度相等,完美二叉树,(2^h -1)个节点            return (1<<rh)-1;        }        //递归求解        return countNodes(root.left)+countNodes(root.right)+1;    }}