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; } }