lc515. 在每个树行中找最大值（队列）

给定一棵二叉树的根节点 root ，请找出该二叉树中每一层的最大值。
示例1：

输入: root = [1,3,2,5,3,null,9]
输出: [1,3,9]
示例2：

输入: root = [1,2,3]
输出: [1,3]

提示：

二叉树的节点个数的范围是 [0,104]
-231 <= Node.val <= 231 - 1

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> largestValues(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if (root == null) return res;
        Deque<TreeNode> q = new LinkedList<>();
        q.addLast(root);
        while (!q.isEmpty()) {
            int size = q.size();
            int max = Integer.MIN_VALUE;
            while (size -- > 0) {
                TreeNode t = q.pollFirst();
                max = Math.max(max, t.val);
                if (t.left != null) q.addLast(t.left);
                if (t.right != null) q.addLast(t.right);
            }
            res.add(max);
        }
        return res;
    }
}