Count Complete Tree Nodes ——LeetCode

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 2hnodes inclusive at the last level h.

题目大意：给定一个完全二叉树，返回这个二叉树的节点数量。

解题思路：一开始想着层次遍历，O(n)，结果TLE，后来优化了下，采用递归的方式来做，总节点数等于1+左子树的数量+右子树的数量，因为是完全二叉树，所以对于子树是满二叉树的可以直接计算出结果并返回。

public class Solution {

    public int countNodes(TreeNode root) {

        if(root==null){

            return 0;

        }

        TreeNode left =root,right=root;

        int cnt = 0;

        while(right!=null){

            left=left.left;

            right=right.right;

            cnt++;

        }

        if(left==null){

            return (1<<cnt)-1;

        }

        return 1+countNodes(root.left)+countNodes(root.right);

    }

}