Rand1 -> RandN

Rand1*1 => uinformly distribution of 0,1 2

Rand1*2 + Rand1 => 0,1,2,3 4

Rand1*3 + Rand2 => 0,1,2,3,4,5 6

Rand1*4 + Rand3 => 0,1,2,3,4,5,6,7 8

...

Rand1*5 + Rand4 => 0,1,2,3,4,5,6,7,8,9 10

if(n==63)

63<64.

Rand1*32 + Rand31 == Rand1*32 + Rand1*31 + Rand30....

RandN(){

if(N==1)

　　return ...

}

int getRandN(int n){
int ret = 0;
while(true){
ret = RandN(n);
if(ret<n)
break;
}
}

int RandN(int n){

if(n<1)
return -1;
if(n==1)
return Rand1();
n2 = n/2;
return Rand1*n2 + RandN(n2-1);

}