LeetCode20|从上到下打印二叉树

例如:
给定二叉树: [3,9,20,null,null,15,7],

    3
   / 
  9  20
    /  
   15   7
返回：

[3,9,20,15,7]

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

public class LevelOrderTest {
    public static void main(String[] args) {
        TreeNode t1 = new TreeNode(3);
        TreeNode t2 = new TreeNode(9);
        TreeNode t3 = new TreeNode(20);
        TreeNode t4 = new TreeNode(15);
        TreeNode t5 = new TreeNode(7);
        t1.left = t2;
        t1.right = t3;
        t3.left = t4;
        t4.right = t5;
        int[] levelOrder = levelOrder(t1);
        for (int j : levelOrder) {
            System.out.print(j + "t");
        }
    }

    public static int[] levelOrder(TreeNode root) {
        if (root == null) {
            return new int[0];
        }
        List<List<Integer>> listList = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            List<Integer> list = new ArrayList<>();
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                list.add(node.val);
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            listList.add(list);
        }
        List<Integer> totalList = new ArrayList<>();
        for (List<Integer> list : listList
        ) {
            totalList.addAll(list);
        }
        int[] result = new int[totalList.size()];
        for (int i = 0; i < totalList.size(); i++) {
            result[i] = totalList.get(i);
        }
        return result;
    }
}