c++ 关键点

INT_MIN、INT_MAX

mid = l + （r -l) / 2

(r -l) /2 向下取整 1/2 = 0,如果向上取整，被除数+1即可， (r -l+1) /2

class Solution {

public:

    vector<vector<int>> levelOrder(TreeNode* root) {

        vector<vector<int>> ret;

        if (root == nullptr) {

            return ret;

        }

        queue <TreeNode*> q;

        q.push(root);

        while (!q.empty()) {

            int n = q.size();

            vector<int> vec;

            for (int i =0; i <n; ++i) {

                TreeNode* t = q.front();

                q.pop();

                vec.push_back(t->val);

                if (t->left) q.push(t->left);

                if (t->right) q.push(t->right);          

            }

            ret.push_back(vec);

        }

        return ret;

    }

};