出處 ： https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=1316
解題策略
p!/(q!(p-q)!)=p(p-1)(p-2)…(p-q+1)/q(q-1)(q-2)…1 => (p-q+i)/i,i=1,2,3,…,q
(s!(r-s)!)/r!=s(s-1)(s-2)…1/r(r-1)(r-2)…(r-s+1) => i/(r-s+i),i=1,2,3,…,s