不分行从上往下打印二叉树

从上往下打印出二叉树的每个结点，同一层的结点按照从左到右的顺序打印。

样例
输入如下图所示二叉树[8, 12, 2, null, null, 6, null, 4, null, null, null]
    8
   / \
  12  2
     /
    6
   /
  4
输出：[8, 12, 2, 6, 4]

BFS

时间复杂度O(n)

class Solution {
    public List<Integer> printFromTopToBottom(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        Queue<TreeNode> q = new LinkedList<>();
        if(root == null){
            return list;
		}
        q.offer(root);
        while(q.size() != 0){
            TreeNode tmp = q.peek();
            q.poll();
            list.add(tmp.val);
            if(tmp.left != null)
                q.offer(tmp.left);
            if(tmp.right != null)
                q.offer(tmp.right);
        }
        return list;
    }
}