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); } }