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.


   / \
  2   3
 / \  /
4  5 6

Output: 6


The below algorithm uses a simple recursion and counts each node in a given binary tree.

func countNodes(root *TreeNode) int {
   var count int
    helper(root, &count)
    return count

func helper(root *TreeNode, count *int){
    if root==nil{
    helper(root.Left, count)
    helper(root.Right, count)


The below code recursively calculates the count without an extra variable.

class Solution {
    public int countNodes(TreeNode root) {
        if(root==null) return 0;
        int left = countNodes(root.left);
        int right = countNodes(root.right);
        return 1+left+right;