LeetCode 257 - Binary Tree Paths

I/P:

19

50 25 12 n n 37 30 n n n 75 62 n 70 n n 87 n n

O/P:

[50->25->12, 50->25->37->30, 50->75->62->70, 50->75->87]

https://leetcode.com/problems/binary-tree-paths/

Input: root = [1,2,3,null,5]
Output: ["1->2->5","1->3"]

package pep.Day32;

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

public class LeetCode_237_Binary_Tree_Paths {
    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int data, TreeNode left, TreeNode right) {
            this.val = data;
            this.left = left;
            this.right = right;
        }
    }

    public static class Pair {
        TreeNode node;
        int state;

        Pair(TreeNode node, int state) {
            this.node = node;
            this.state = state;
        }
    }

    public static TreeNode construct(Integer[] arr) {
        TreeNode root = new TreeNode(arr[0], null, null);
        Pair rtp = new Pair(root, 1);

        Stack<Pair> st = new Stack<>();
        st.push(rtp);

        int idx = 0;
        while (st.size() > 0) {
            Pair top = st.peek();
            if (top.state == 1) {
                idx++;
                if (arr[idx] != null) {
                    top.node.left = new TreeNode(arr[idx], null, null);
                    Pair lp = new Pair(top.node.left, 1);
                    st.push(lp);
                } else {
                    top.node.left = null;
                }

                top.state++;
            } else if (top.state == 2) {
                idx++;
                if (arr[idx] != null) {
                    top.node.right = new TreeNode(arr[idx], null, null);
                    Pair rp = new Pair(top.node.right, 1);
                    st.push(rp);
                } else {
                    top.node.right = null;
                }

                top.state++;
            } else {
                st.pop();
            }
        }

        return root;
    }

    public static void display(TreeNode node) {
        if (node == null) {
            return;
        }

        String str = "";
        str += node.left == null ? "." : node.left.val + "";
        str += " <- " + node.val + " -> ";
        str += node.right == null ? "." : node.right.val + "";
        System.out.println(str);

        display(node.left);
        display(node.right);
    }

    public static List<String> binaryTreePaths(TreeNode root) {
        List<String> ans = new ArrayList<>();
        helper(root, "", ans);
        return ans;
    }

    public static void helper(TreeNode root, String ans, List<String> al) {
        if (root == null)
            return;
        else if (root.left == null && root.right == null) {
            ans += root.val;
            al.add(ans);
            return;
        }

        helper(root.left, ans + root.val + "->", al);
        helper(root.right, ans + root.val + "->", al);

    }

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

        int data = Integer.parseInt(br.readLine());

        TreeNode root = construct(arr);

        System.out.println(binaryTreePaths(root));

    }

}