Remove Leaves In 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 -> . 30 -> 80, . 80 -> . 40 -> .

package pep.Day29;

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

public class Remove_Leaves_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 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) {
        // jine child ke children nhi hai
        // remove them
        for (int i = node.children.size() - 1; i >= 0; i--) {
            Node child = node.children.get(i);
            if (child.children.isEmpty())
                node.children.remove(i);
        }

        // make recursive call for all child
        for (Node child : node.children) {
            removeLeaves(child);
        }
    }

    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);
        removeLeaves(root);
        display(root);
    }

}

Time Complexity: We are traversing all the nodes once, hence the time complexity will be O(n) where n = number of nodes in the generic tree. 

Space Complexity: We are not using any extra space in the form of any auxiliary data structure. Hence the space complexity is O(1). Note: We are using recursion which does take stack space of O(d) where d = maximum depth of the generic tree.