//int31n也就是下面这个函数,跟上面Int31n效果是一样的.但是效率更高.算法不一样.这个算法非常精彩,效率也更高.//int31nreturns,asanint32,anon-negativepseudo-randomnumberinthehalf-openinterval[0,n).//nmustbe>0,butint31ndoesnotcheckthis;