Java实现二叉树层次遍历并存入List的方法：从上往下,从左往右

import java.util.ArrayList;
import java.util.Queue;
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 {
    //方法主体
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<>();

        if (null == root) {
            return list;
        }
              
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (queue.size() != 0) {
            TreeNode tmp = queue.poll();
            list.add(tmp.val);
            if (null != tmp.left) {
                queue.add(tmp.left);
            }
            if (null != tmp.right) {
                queue.add(tmp.right);
            }
        }
        return list;
    }
}                

 Queue队列的用法详见：https://www.runoob.com/java/data-queue.html