题目：

题解：

class Solution {
public:
    vector> levelOrder(TreeNode* root) {
        queue que;
        vector> res;
        if (root != nullptr) que.push(root);
        while (!que.empty()) {
            vector tmp;
            for(int i = que.size(); i > 0; --i) {
                root = que.front();
                que.pop();
                tmp.push_back(root->val);
                if (root->left != nullptr) que.push(root->left);
                if (root->right != nullptr) que.push(root->right);
            }
            res.push_back(tmp);
        }
        return res;
    }
};