srand( (unsigned)time( NULL ) );
rand() % N;

 R = RAND_MAX-(RAND_MAX+1)%N; //去除尾数
  t = rand();
  while( t > R ) t = rand();
  result = t % N; // 符合要求的随机数

 r = N % (RAND_MAX+1); //余数
  if ( happened( (double)r/N ) )//选到余数段的概率
  result = N-r+myrandom(r); // myrandom可以用情况1中的代码实现
  else
  result = rand()+myrandom(N/(RAND_MAX+1))*(RAND_MAX+1); // 如果选不到余数段再进行分段选取

#include<iostream.h>
#include<time.h>
#include<stdlib.h>
const double MinProb=1.0/(RAND_MAX+1);
bool happened(double probability)//probability 0~1
{
 if(probability<=0)
 {
return false;
 }
 if(probability<MinProb)
 {
 return rand()==0&&happened(probability*(RAND_MAX+1));
 }
 if(rand()<=probability*(RAND_MAX+1))
 {
 return true;
 }
 return false;
}
long myrandom(long n)//产生0~n-1之间的等概率随机数
{
 t=0;
 if(n<=RAND_MAX)
 {
 long R=RAND_MAX-(RAND_MAX+1)%n;//尾数
 t = rand();
 while ( t > r )
 {
  t = rand();
 }
 return t % n;
 }
 else
 {
 long r = n%(RAND_MAX+1);//余数
 if( happened( (double)r/n ) )//取到余数的概率
 {
  return n-r+myrandom(r);
 }
 else
 {
  return rand()+myrandom(n/(RAND_MAX+1))*(RAND_MAX+1);
 }
 }
}
 
还有另外一种非常简单的方式，那就是使用
random_shuffle( RandomAccessIterator _First, RandomAccessIterator _Last ).
例如，生成0——N-1之间的随机数，可以这么写
#include <algorithm>
#include <vector>
long myrandom( long N )
{
 std::vector<long> vl( N ); // 定义一个大小为N的vector
 for ( long i=0; i<N; ++i )
 {
  vl[i] = i;
 }
 std::random_shuffle( vl.begin(), vl.end() );
 return (*vl.begin());
}
random_shuffle 还有一个三参数的重载版本
random_shuffle( RandomAccessIterator _First, RandomAccessIterator _Last, RandomNumberGenerator& _Rand )