//level order traversal ---->  lecture 8 striver 

#include <vector>
#include <queue>

using namespace std;

// Definition for a binary tree node.
struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;

    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> ans;

        if (root == nullptr)
            return ans;

        queue<TreeNode*> q;
        q.push(root);

        while (!q.empty()) {
            int size = q.size();
            vector<int> level;

            for (int i = 0; i < size; i++) {
                TreeNode* node = q.front();
                q.pop();

                if (node->left != nullptr)
                    q.push(node->left);
                if (node->right != nullptr)
                    q.push(node->right);

                level.push_back(node->val);
            }

            ans.push_back(level);
        }

        return ans;
    }
};

//iterative preorder traversal