题目：

题解：

class Solution {
public:
    vector<int> largestValues(TreeNode* root) {
        if (!root) {
            return {};
        }
        vector<int> res;
        queue<TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            int len = q.size();
            int maxVal = INT_MIN;
            while (len > 0) {
                len--;
                auto t = q.front();
                q.pop();
                maxVal = max(maxVal, t->val);
                if (t->left) {
                    q.push(t->left);
                }
                if (t->right) {
                    q.push(t->right);
                }
            }
            res.push_back(maxVal);
        }
        return res;
    }
};