LeetCode: 102. Binary Tree Level Order Traversal

https://leetcode.com/problems/binary-tree-level-order-traversal/

class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int> > ret;
        if(root == NULL)
            return ret;

        queue<TreeNode *> Q;
        Q.push(root);
        while(!Q.empty()) {
        	vector<int> sublist;
        	int levelNum = Q.size();
        	for (int i = 0; i < levelNum; i++) {
        		TreeNode *cur = Q.front();
        		if (cur->left != NULL)
        			Q.push(cur->left);
        		if (cur->right != NULL)
        			Q.push(cur->right)
        		sublist.push_back(cur->val);
        		Q.pop();
        	}
        	ret.push_back(sublist);
        }

        return ret;
    }
};