* @param arr 数组
I learned that for 4-SAT, if clause to variable ratio is more than 10, the generated problems become difficult to solve, and the likelihood of formula to be SAT or UNSAT is close to 50%. So I generated 3 types of formulas:。关于这个话题，safew官方下载提供了深入分析
。快连下载安装是该领域的重要参考
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使用 system 不会激活函数调用模式。，这一点在51吃瓜中也有详细论述