网上偶然看到一种更高效的判断质数的方法
首先看一个关于质数分布的规律：
大于等于5的质数一定和6的倍数相邻。例如5和7，11和13,17和19等等
证明：
令x≥1，将大于等于5的自然数表示如下：
······ 6x-1，6x，6x+1，6x+2，6x+3，6x+4，6x+5，6(x+1），6(x+1)+1 ······
可以看到，不在6的倍数两侧，即6x两侧的数为:
6x+2，6x+3，6x+4，由于2(3x+1)，3(2x+1)，2(3x+2)
所以它们一定不是素数，再除去6x本身
显然，素数要出现只可能出现在6x的相邻两侧。
根据以上规律，判断质数可以6个为单元快进
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