剑指 Offer 32 - I. 从上到下打印二叉树

剑指 Offer 32 - I. 从上到下打印二叉树

难度中等28收藏分享切换为英文关注反馈

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]

/**
     * https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-lcof/
     * 剑指 Offer 32 - I. 从上到下打印二叉树
     *
     * @param root
     * @return
     */
    public int[] levelOrder(TreeNode root) {
        if (root == null) {
            return new int[0];
        }
        List<Integer> list = new ArrayList<>();
        Deque<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        list.add(root.val);
        while (!queue.isEmpty()) {
            TreeNode poll = queue.poll();
            if (poll.left != null) {
                queue.add(poll.left);
                list.add(poll.left.val);
            }
            if (poll.right != null) {
                queue.add(poll.right);
                list.add(poll.right.val);
            }
        }
        int[] res = new int[list.size()];
        for (int i = 0; i < list.size(); i++) {
            res[i] = list.get(i);
        }
        return res;
    }