@inproceedings{shujun2008the, title={THE RESEARCH ON THE DIFFICULTY GRADE AND PIECING SKILLS OF TANGRAM}, author={Shujun Cai, Yizhi Jiang, and Minyu Zeng}, url={http://archive.ymsc.tsinghua.edu.cn/pacm_paperurl/20160813215154756648063}, year={2008}, }