力扣 LeetCode 102. 二叉树的层序遍历（Day6：二叉树）

解题思路：

使用队列

class Solution {public List<List<Integer>> levelOrder(TreeNode root) {Deque<TreeNode> queue = new ArrayDeque<>();List<List<Integer>> res = new ArrayList<>();if (root == null) return res;queue.add(root);while (!queue.isEmpty()) {int size = queue.size();List<Integer> list = new ArrayList<>();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);}res.add(list);}return res;}
}