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/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<vector<int>> levelOrder(TreeNode* root) {
        vector<vector<int>> res;
        if (!root) return res;
        queue<TreeNode*> Q;
        Q.push(root);
        while (!Q.empty()) {
            int n = Q.size();
            vector<int> v;
            for (int i = 0; i < n; i++) {
                TreeNode* p = Q.front(); Q.pop();
                v.push_back(p -> val);
                if (p -> left) Q.push(p -> left);
                if (p -> right) Q.push(p -> right);
            }
            res.push_back(v);
        }
        return res;
    }
};