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) {
	  //使用层序遍历
        Queue<TreeNode> queue = new ArrayDeque<>();
        ArrayList<Integer> list = new ArrayList<>();
        if (root == null) {
            return list;
        }
	  //层序遍历结果加入list
        queue.add(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode tmp = queue.poll();
                list.add(tmp.val);
                if (tmp.left != null) {
                    queue.add(tmp.left);
                }
                if (tmp.right != null) {
                    queue.add(tmp.right);
                }
            }
        }



        return list;
    }
}