Mirror Of A Generic Tree

I/P:

24 10 20 50 -1 60 -1 -1 30 70 -1 80 110 -1 120 -1 -1 90 -1 -1 40 100 -1 -1 -1

O:P:

10 -> 20, 30, 40, . 20 -> 50, 60, . 50 -> . 60 -> . 30 -> 70, 80, 90, . 70 -> . 80 -> 110, 120, . 110 -> . 120 -> . 90 -> . 40 -> 100, . 100 -> .

10 -> 40, 30, 20, . 40 -> 100, . 100 -> . 30 -> 90, 80, 70, . 90 -> . 80 -> 120, 110, . 120 -> . 110 -> . 70 -> . 20 -> 60, 50, . 60 -> . 50 -> .

package pep.Day29;

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

public class Mirror_A_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 void mirror(Node node) {
        // faith: each children will bring the reverse child of their own
        for (Node child : node.children)
            mirror(child);
        // expectation meet faith: reverse the childern arraylist 
        Collections.reverse(node.children);
    }

    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);
        display(root);
        mirror(root);
        display(root);
    }

}

Time Complexity: O(n) The time complexity for the function is linear as we post traversing the tree.

Space Complexity: O(nlogn) The space complexity for the function is equal to the height of the tree due to the recursion stack.