题目：
STINGY SAT is the following problem: given a set of clauses (each a disjunction of literals) and an integer k, find a satisfying assignment in which at most k variables are true, if such an assignment exists. Prove that STINGY SAT is NP-complete.
solution:
显然STINGY SAT的解是可在多项式时间内验证的
故STINGY SAT的解属于NP
且显然将k设为所有变量的总个数，可以将SAT归约到STINGY SAT
所以
STINGY SAT为NP完全问题