https://leetcode.com/problems/count-complete-tree-nodes/description/

NodeCounter.java

class NodeCounter {

    int counter = 0;
    public static void main(String[] args) {
        TreeNode node4 = new TreeNode(4);
        TreeNode node5 = new TreeNode(5);
        TreeNode node6 = new TreeNode(6);
        TreeNode node2 = new TreeNode(2, node4, node5);
        TreeNode node3 = new TreeNode(3, node6, null);
        TreeNode node1 = new TreeNode(1, node2, node3);
        System.out.println(new NodeCounter().countNodes(node1));
    }

    public int countNodes(TreeNode root) {
        if(root!=null){
            counter++;
            countNodes(root.left);
            countNodes(root.right);
        }
        return counter;
    }
}

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;
    }
}