515. 在每个树行中找最大值

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]

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> largestValues(TreeNode* root) {
        queue<TreeNode*>que;
        vector<int>res;

        if(root == nullptr) return res;
        que.push(root);
        while(!que.empty()){
            int size = que.size();
            int maxnum = INT32_MIN;
            while(size--){
                TreeNode* node = que.front();
                que.pop();
                if(node->val > maxnum) maxnum = node->val;
                if(node->left) que.push(node->left);
                if(node->right) que.push(node->right);
            }
            res.push_back(maxnum);
        }
        return res;
    }
};