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// in pre-order
void serialize_btree(TreeNode *root, string &s) {
	if(!root) {
		s += '#';
		return;
	}
	s += to_string(root->data);
	serialize_btree(root->left, s);
	serialize_btree(root->right, s);
}

// in pre-order
TreeNode* deserialize_btree(string &s, int &curr) {
	if(curr > s.size()) return NULL;
	if(s[curr] == '#') {
		curr++;
		return NULL;
	}
	TreeNode *root = new TreeNode(s[curr++]);
	root->left = deserialize_btree(s, curr);  // gist, this "curr" is different from the below "curr", since we use reference
	root->right = deserialize_btree(s, curr); // gist, this "curr" is different from the above "curr"
	return root;
}