题干：

代码：

/**
 * 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 != NULL) que.push(root);
        while(!que.empty()){
            int size = que.size();
            int max = INT_MIN;
            while(size--){
                TreeNode* node = que.front();
                que.pop();
                max = (node -> val) > max ? (node -> val) : max;
                if(node -> left) que.push(node -> left);
                if(node -> right) que.push(node -> right);
            }
            res.push_back(max);
        }
        return res;


    }
};

代码中出现了这一行：int max = INT_MIN;

INT_MIN被默认为计算机能想到的最小的值，常用于多个元素比较取最大值。