LeetCode之Binary Tree Level Order Traversal

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int> > res;
        if(root == nullptr) return res;
        queue<TreeNode*> q0;
        queue<TreeNode*> q1;
        q0.push(root);
        while(!q0.empty()){
            vector<int> tmp;
            while(!q0.empty()){//出队
                TreeNode *p = q0.front();
                q0.pop();
                tmp.push_back(p->val);
                if(p->left) q1.push(p->left);
                if(p->right) q1.push(p->right);
            }
            res.push_back(tmp);
            swap(q0, q1);
        }
        return res;
    }
};