> 
> 详细说明见：http://www.cppblog.com/flyinghearts/archive/2010/08/02/118593.html
> 
> 算法：
> 　　F(n+2) = F(n+1) + F(n)　　
>    用归纳法可证明：F(n) = F(k)*F(n+1-k) + F(k-1)*F(n-k)
> 　　利用这两个公式可实现O(log n)。
> 
> unsigned fib_last4( unsigned num)
> {
>   if ( num == 0 ) return 0;
>   const unsigned M=10000;
>   unsigned ret=1,next=1,ret_=ret;
>   unsigned flag=1, tt=num;
>   while ( tt >>= 1) flag <<= 1;
>   while ( flag >>= 1 ){
>     if ( num & flag ){
>       ret_ = ret * ret + next * next;
>       next = (ret + ret + next) * next;
>     } else {
>       //多加一个M，避免 2*next-ret是负数，造成结果不对
>       ret_ = (next + next + M - ret) * ret;
>       next = ret * ret + next * next;
>     }
>     ret = ret_ % M;
>     next = next % M;
>   }
>   return ret;
> }