Count Complete Tree Nodes
Given a complete binary tree, count the number of nodes. Note: 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 2 h nodes inclusive at the last level h. Example: Input: 1 / \ 2 3 / \ / 4 5 6 Output: 6 /** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode() {} * TreeNode(int val) { this.val = val; } * TreeNode(int val, TreeNode left, TreeNode right) { * this.val = val; * this.left = left; * this.right = right; * } * } */ class Solution { public int countNodes(TreeNode root) { if(root == null){ return 0; } return 1 + countNodes(root.left) + countNodes(root.right); } } Try it on Leetcode Here, w