剑指 Offer 32 - II. 从上到下打印二叉树 II

从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行。

例如:
给定二叉树: [3,9,20,null,null,15,7],

返回其层次遍历结果：

[
[3],
[9,20],
[15,7]
]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        Deque<TreeNode> queue = new ArrayDeque<>();
        List<List<Integer>> res = new ArrayList<>();
        if (root != null) {
            queue.offer(root);
        }
        while (!queue.isEmpty()) {
            List<Integer> list = new ArrayList<>();
            // for循环从高到低，避免size的大小改变
            for (int i = queue.size(); i > 0; i--) {
                TreeNode node = node = queue.poll();
                list.add(node.val);
                if (node.left != null) {
                queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            res.add(list);
        }
        return res;
    }
}