102.二叉树的层序遍历

给你一个二叉树，请你返回其按 层序遍历 得到的节点值。 （即逐层地，从左到右访问所有节点）

class TreeNode
{
public:
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode():val(NULL),left(nullptr),right(nullptr){}
    TreeNode(int x):val(x),left(nullptr),right(nullptr){}
};

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