linearizeaGT.java

import java.io.*;
import java.util.*;

public class linearizeaGT {
    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 int max(Node node) {
        int m = Integer.MIN_VALUE;

        for (Node child: node.children) {
            int cm = max(child);
            m = Math.max(m, cm);
        }
        m = Math.max(m, node.data);

        return m;
    }

    public static int height(Node node) {
        int h = -1;

        for (Node child: node.children) {
            int ch = height(child);
            h = Math.max(h, ch);
        }
        h += 1;

        return h;
    }

    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 levelOrderLinewiseZZ(Node node) {
        Stack < Node > stack = new Stack < > ();
        stack.add(node);

        Stack < Node > cstack = new Stack < > ();
        int level = 0;

        while (stack.size() > 0) {
            node = stack.pop();
            System.out.print(node.data + " ");

            if (level % 2 == 0) {
                for (int i = 0; i < node.children.size(); i++) {
                    Node child = node.children.get(i);
                    cstack.push(child);
                }
            } else {
                for (int i = node.children.size() - 1; i >= 0; i--) {
                    Node child = node.children.get(i);
                    cstack.push(child);
                }
            }

            if (stack.size() == 0) {
                stack = cstack;
                cstack = new Stack < > ();
                level++;
                System.out.println();
            }
        }
    }

    public static void mirror(Node node) {
        for (Node child: node.children) {
            mirror(child);
        }
        Collections.reverse(node.children);
    }

    public static void removeLeaves(Node node) {
        for (int i = node.children.size() - 1; i >= 0; i--) {
            Node child = node.children.get(i);
            if (child.children.size() == 0) {
                node.children.remove(i);
            }
        }

        for (Node child: node.children) {
            removeLeaves(child);
        }
    }
    public static Node getTail(Node temp){
        while(temp.children.size() > 0){
            temp = temp.children.get(0); // same as temp = temp.next
        }
        return temp;
    }
    

    public static void linearize(Node node) {
        
        for(int i = 0; i < node.children.size(); i++){
            linearize(node.children.get(i));    //faith
        }
        
        while(node.children.size() > 1){
            int size = node.children.size();
            Node sltail = getTail(node.children.get(size - 2)); //size-2 gives the second last index
            sltail.children.add(node.children.remove(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);
        linearize(root);
        display(root);
    }

}