题解 | #从上往下打印二叉树#

import java.util.*;
/**
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) {
        if (root == null) {
            return new ArrayList<Integer>();
        }
       ArrayList<Integer> arr = new ArrayList<Integer>();
        TreeNode node = root;
        LinkedList<TreeNode> list = new LinkedList<TreeNode>();
        list.add(root);
        while (list.size() > 0) {
            TreeNode n = list.removeFirst();
            if(n.left!=null){
                list.addLast(n.left);
            }
            if(n.right!=null){
                list.addLast(n.right);
            }
             arr.add(n.val);

        }
        return arr;

    }
}

形成队列，进来的出队列要把左右子树存入队列，实现广度优先