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;
    }
}