层序遍历

    vector<vector<int> > Print(TreeNode* pRoot) {
        // write code here
        if(pRoot == nullptr) return {};
        vector<vector<int>> res;
        queue<TreeNode*> Q;
        Q.push(pRoot);
        vector<int> tmp;
        while(!Q.empty()){
            tmp.clear();
            int len = Q.size();

            for(int i = 0; i < len; ++i){
                TreeNode* cur = Q.front();
                Q.pop();
                if(cur != nullptr){
                    tmp.emplace_back(cur->val);
                    if(cur->left) Q.push(cur->left);
                    if(cur->right) Q.push(cur->right);
                }
            }
            res.emplace_back(tmp);
        }
        return res;
    }