Left View of Binary Tree

// { Driver Code Starts
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 GfG {
    
    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--;
        }
    }
}
// } Driver Code Ends


//User function Template for Java

/* A Binary Tree node
class Node
{
    int data;
    Node left, right;

    Node(int item)
    {
        data = item;
        left = right = null;
    }
}*/
class Tree
{
    ArrayList<Integer> leftView(Node root)
    {
    //存结果
      ArrayList<Integer> result = new ArrayList<>();
      if(root == null) {
          return result;
      }
      //队列做数据结构
      Queue<Node> queue = new ArrayDeque<>();
      queue.add(root);
      
      
      
      
      while(!queue.isEmpty()) {
          int size = queue.size();
          int i = 0;
          
          //换层后queue的计数为当层size
          while(i++ < size) {
             Node curr = queue.poll();
              
              //每层第一个节点开始i都是1
              if(i == 1) {
                  result.add(curr.data);
              }
              //将当前节点，包括该层的第一个及后面的左右子节点都加入队列
              if(curr.left != null) {
                  queue.add(curr.left);
              }
              if(curr.right != null) {
                  queue.add(curr.right);
              }
          }
      }
      return result;
    }
}

打印右边的第一个节点则先加入右子节点，再加入左子节点。