/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */


class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Level order a list of lists of integer
     */
public:
    vector<vector<int>> levelOrder(TreeNode *root) {
        // write your code here
        vector<vector<int>> results;
        if (root == NULL) {
            return results;
        }

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

        while (!q.empty()) {
            int size = q.size();
            vector<int> level;
            // work on current level nodes by stuffing nodes from next level into the queue;
            for (int i = 0; i < size; i++) {
                TreeNode* cur = q.front();
                q.pop();
                level.push_back(cur->val);
                if (cur->left != NULL) {
                    q.push(cur->left);
                }
                if (cur->right != NULL) {
                    q.push(cur->right);
                }
            }
            results.push_back(level);
        }

        return results;
    }
};