JZ22 从上往下打印二叉树

从上往下打印出二叉树的每个节点，同层节点从左至右打印。

https://www.nowcoder.com/practice/7fe2212963db4790b57431d9ed259701

import java.util.ArrayList;
import java.util.LinkedList;
/**
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<>();
        if (root == null) return result;
        
        // LinkedList 继承自队列接口
        LinkedList<TreeNode> queue = new LinkedList<>();
        
        queue.add(root);
        while(!queue.isEmpty()) {
            TreeNode temp = queue.pop();
            if (temp.left != null) {
                queue.add(temp.left);
            }
            
            if (temp.right != null) {
                queue.add(temp.right);
            }
            result.add(temp.val);
        }
        
        return result;
    }
}