这个笔记记录高效的决策树系列算法，主要阅读论文，结合一些开源框架，希望在弄清算法的基础上，尝试着改进算法，尝试着工程实现．
我们知道，目前比较流行的两个GBDT开源框架是XGBoost和LightGBM，无论内存占用还是计算速度，它们都做到了淋漓尽致．在LightGBM的Feature上提到了，XGBoost的decision tree用的是pre-sorted based的算法，也就是在tree building之前对各维特征先排序，代表性的算法是SLIQ[1]和SPRINT[2]．而LightGBM的decision tree是histogram based的算法，也就是先将特征离散化，代表性的算法是CLOUDS[3],Mcrank[4]和Machado[5]．
SLIQ和SPRINT算法的特点决定了树生长的方式是level-wise(breadth-first)的，与之对应的是leaf-wise(depth-wise，best-wise[6])的方式，LightGBM正是采用leaf-wise的方式．
内容大致按以下几部分展开:
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基于特征预排序的算法SLIQ
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基于特征预排序的算法SPRINT
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基于特征离散化的算法CLOUDS
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研究开源框架: LightGBM，XGBoost
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自己动手实现一个高效决策树 tgboost
[1] Mehta, Manish, Rakesh Agrawal, and Jorma Rissanen. “SLIQ: A fast scalable classifier for data mining.” International Conference on Extending Database Technology. Springer Berlin Heidelberg, 1996.
[2] Shafer, John, Rakesh Agrawal, and Manish Mehta. “SPRINT: A scalable parallel classifier for data mining.” Proc. 1996 Int. Conf. Very Large Data Bases. 1996.
[3] Ranka, Sanjay, and V. Singh. “CLOUDS: A decision tree classifier for large datasets.” Proceedings of the 4th Knowledge Discovery and Data Mining Conference. 1998.
[4] Machado, F. P. “Communication and memory efficient parallel decision tree construction.” (2003).
[5] Li, Ping, Qiang Wu, and Christopher J. Burges. “Mcrank: Learning to rank using multiple classification and gradient boosting.” Advances in neural information processing systems. 2007.
[6] Shi, Haijian. “Best-first decision tree learning.” Diss. The University of Waikato, 2007.