剑指 Offer 32 - I. 从上到下打印二叉树.从左到右顺序打印

题目：

面试遇到过，重点

剑指 Offer 32 - I. 从上到下打印二叉树
从上到下打印出二叉树的每个节点，同一层的节点按照从左到右的顺序打印。
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]
提示：节点总数 <= 1000
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-lcof

解题：

关键点：层次遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
     
    public int[] levelOrder(TreeNode root) {
     
        if (root == null) {
     
            return new int[]{
     };
        }
        List<Integer> result = new ArrayList<>();

        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
     
            TreeNode tem = queue.poll();
            result.add(tem.val);
            if (tem.left != null) {
     
                queue.offer(tem.left);
            }
            if (tem.right != null) {
     
                queue.offer(tem.right);
            }
        }
        int[] result_arr = new int[result.size()];
        for (int i = 0; i < result.size(); i ++) {
     
            result_arr[i] = result.get(i);
        }
        return result_arr;
    }
}