102. Binary Tree Level Order Traversal

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> ans;
        vector<int>ans2;
        if (root==NULL) return {};
        
        queue<TreeNode*> q;
        q.push(root);
        q.push(NULL);
        while(!q.empty()){
            TreeNode* front = q.front();
            q.pop();
            
            if(front==NULL) {
                ans.push_back(ans2);
                ans2.clear();
                if(!q.empty()) {
                    q.push(NULL);
                }
            }
            else{
                ans2.push_back(front->val);
                if(front->left) q.push(front->left);
                if(front->right) q.push(front->right);
            }            
        }
        return ans;
    }
};