一个简单有效的产生随机数且不重复的小算法--Robert Floyd

initialize set S to empty
for J := N-M + 1 to N do
T := RandInt(1, J)
if T is not in S then
insert T in S
else
insert J in S

这里N是随机数范围的最大值，M是要选取的随机数的数量

伪代码来源：Jon Bentley's Programming Pearls column "A sample of Brilliance"