剑指offer32-II.从上到下打印二叉树（二维数组输出）

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        if (root == NULL) return ans;
        queue<TreeNode*> q;
        q.push(root);
        ans.push_back({root->val});
        while(!q.empty()) {
            int n = q.size();
            vector<int> vec;
            for (int i = 0; i < n; i++) {
                auto p = q.front();
                if (p->left) {
                    vec.push_back(p->left->val);
                    q.push(p->left);
                }
                if (p->right) {
                    vec.push_back(p->right->val);
                    q.push(p->right);
                }
                q.pop();
            }
            if (vec.size()) ans.push_back(vec);
        }
        return ans;
    }
};