Alpha-Beta剪枝
https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-034-artificial-intelligence-fall-2010/lecture-videos/lecture-6-search-games-minimax-and-alpha-beta/ 21：00开始
PURNING：
While examining the children of a maximizer, if v of maximizer > beta, prune the rest of the children.
While examining the children of a minimizer, if v of minimizer < alpha, prune the rest of the children
UPDATE:
Max: alpha; Min: beta.
