题解 | #从上往下打印二叉树#

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.Queue;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        
        ArrayList<Integer> result = new ArrayList<Integer>();
        if(root == null){
            return result;
        }
        
        
        Queue<TreeNode> queue = new LinkedList<>();
        
        result.add(root.val);
        if(root.left != null){
            queue.offer(root.left);
        }
        if(root.right != null){
            queue.offer(root.right);
        }
        
        if(!queue.isEmpty()){
            collect(result,queue);
        }
        
        return result;
        
    }
    
    public void collect(ArrayList<Integer> result, Queue<TreeNode> queue){
        while(queue.peek() != null){
            TreeNode node = queue.poll();
            
            result.add(node.val);
            if(node.left != null){
                queue.offer(node.left);
            }
            if(node.right != null){
                queue.offer(node.right);
            }

            if(!queue.isEmpty()){
                collect(result,queue);
            }
        }
    }
}