Generic Tree - Traversals (pre-order, Post-order) / DEPTH FIRST TRAVERSAL

I/O:

12 10 20 -1 30 50 -1 60 -1 -1 40 -1 -1

O/P:

Node Pre 10 Edge Pre 10--20 Node Pre 20 Node Post 20 Edge Post 10--20 Edge Pre 10--30 Node Pre 30 Edge Pre 30--50 Node Pre 50 Node Post 50 Edge Post 30--50 Edge Pre 30--60 Node Pre 60 Node Post 60 Edge Post 30--60 Node Post 30 Edge Post 10--30 Edge Pre 10--40 Node Pre 40 Node Post 40 Edge Post 10--40 Node Post 10

Pre - Order Traversal-

● When we are anywhere to the left of an element, it is called Pre - Order Traversal.

● It is written before the recursive call is made.

● This takes place while going up the stack.

● Root is visited before visiting its children.

Post - Order Traversal-

● Whenever we are at the right of an element, it is called Post - Order Traversal.

● It is written after the recursive call is made.

● This takes place while going down the stack.

● Root is visited after visiting its children.

package pep.Day28;

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

public class Traversals_In_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) {
        int s = 0;

        for (Node child : node.children) {
            s += size(child);
        }
        s += 1;

        return s;
    }

    public static void traversals(Node node) {
        System.out.println("Node Pre " + node.data);
        for (Node child : node.children) {
            System.out.println("Edge Pre " + node.data + "--" + child.data);
            traversals(child);
            System.out.println("Edge Post " + node.data + "--" + child.data);
        }
        System.out.println("Node Post " + node.data);

    }

    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);
        traversals(root);
    }

}

Time Complexity: O(2n)
Space Complexity: O(1)