Left view of a Binary Tree is set of nodes visible when tree is visited from left side.

LeftViewBinaryTree.java

package GeeksDailyProblem;

import java.util.LinkedList;
import java.util.Queue;
import java.io.*;
import java.util.*;

class Node {
    int data;
    Node left;
    Node right;

    Node(int data) {
        this.data = data;
        left = null;
        right = null;
    }
}

class LeftViewBinaryTree {

    static Node buildTree(String str) {

        if (str.length() == 0 || str.charAt(0) == 'N') {
            return null;
        }

        String ip[] = str.split(" ");
        // Create the root of the tree
        Node root = new Node(Integer.parseInt(ip[0]));
        // Push the root to the queue

        Queue<Node> queue = new LinkedList<>();

        queue.add(root);
        // Starting from the second element

        int i = 1;
        while (queue.size() > 0 && i < ip.length) {

            // Get and remove the front of the queue
            Node currNode = queue.peek();
            queue.remove();

            // Get the current node's value from the string
            String currVal = ip[i];

            // If the left child is not null
            if (!currVal.equals("N")) {

                // Create the left child for the current node
                currNode.left = new Node(Integer.parseInt(currVal));
                // Push it to the queue
                queue.add(currNode.left);
            }

            // For the right child
            i++;
            if (i >= ip.length)
                break;

            currVal = ip[i];

            // If the right child is not null
            if (!currVal.equals("N")) {

                // Create the right child for the current node
                currNode.right = new Node(Integer.parseInt(currVal));

                // Push it to the queue
                queue.add(currNode.right);
            }
            i++;
        }

        return root;
    }

    static void printInorder(Node root) {
        if (root == null)
            return;

        printInorder(root.left);
        System.out.print(root.data + " ");

        printInorder(root.right);
    }

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

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

        while (t > 0) {
            String s = br.readLine();
            Node root = buildTree(s);
            Tree g = new Tree();
            ArrayList<Integer> result = g.leftView(root);
            for (int value : result) {
                System.out.print(value + " ");
            }
            System.out.println();
            t--;
        }
    }
}

class Tree {
    //Function to return list containing elements of left view of binary tree.
    static int maxLevel = 0;
    ArrayList<Integer> leftViewData = new ArrayList<>();

    ArrayList<Integer> leftView(Node root) {
        maxLevel = 0;
        leftViewUtil(root, 1);
        return leftViewData;
    }

    private void leftViewUtil(Node root, int level) {
        if (root == null)
            return;

        if (maxLevel < level) {
            leftViewData.add(root.data);
            maxLevel = level;
        }

        leftViewUtil(root.left, level + 1);
        leftViewUtil(root.right, level + 1);
    }
}