Size of Generic Tree

I/P: 

12

10 20 -1 30 50 -1 60 -1 -1 40 -1 -1

O/P: 6

Expectation:

We will have an expectation that sizeOfTree(10) will return as the size of the tree rooted at 10.

Faith:

We already have faith that sizeOfTree(20), sizeOfTree(30), sizeOfTree(40) will return us the size of the respective trees. Now to establish the expectation from the faith what we have to do is:

sizeOfTree(root) = sum of sizeOfTree(child) + 1

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.Stack;

public class Size_Of_GenericTree {

    private static class Node {
        int data;
        ArrayList<Node> children = new ArrayList<>();
    }

    public static void display(Node node) {
        String str = node.data + " -> ";
        for (Node child : node.children) {
            str += child.data + ", ";
        }
        str += ".";
        System.out.println(str);

        for (Node child : node.children) {
            display(child);
        }
    }

    public static Node construct(int[] arr) {
        Node root = null;

        Stack<Node> st = new Stack<>();
        for (int i = 0; i < arr.length; i++) {
            if (arr[i] == -1) {
                st.pop();
            } else {
                Node t = new Node();
                t.data = arr[i];

                if (st.size() > 0) {
                    st.peek().children.add(t);
                } else {
                    root = t;
                }

                st.push(t);
            }
        }

        return root;
    }

    public static int size(Node node) {
        // write your code here
        if (node == null) return 0;
        int size = 0;
        // Expectation: children apna size khud calculate kr lenge
        // let 20,30,40 apni apni node ka count le aate hain
        ArrayList<Node> children = node.children;
        for (Node child : children)
            size += size(child);

        // end me 10 (parent) apna count +1 kr dega
        return size + 1;
    }


    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(br.readLine());
        int[] arr = new int[n];
        String[] values = br.readLine().split(" ");
        for (int i = 0; i < n; i++) {
            arr[i] = Integer.parseInt(values[i]);
        }

        Node root = construct(arr);
        int sz = size(root);
        System.out.println(sz);
        // display(root);
    }

}