阶乘最后非零位

//求阶乘最后非零位,复杂度O(nlogn)//返回该位,n以字符串方式传入#include <string.h>#define MAXN 10000int lastdigit(char* buf){const int mod[20]={1,1,2,6,4,2,2,4,2,8,4,4,8,4,6,8,8,6,8,2};int len=strlen(buf),a[MAXN],i,c,ret=1;if (len==1)return mod[buf[0]-'0'];for (i=0;i<len;i++)a[i]=buf[len-1-i]-'0';for (;len;len-=!a[len-1]){ret=ret*mod[a[1]%2*10+a[0]]%5;for (c=0,i=len-1;i>=0;i--)c=c*10+a[i],a[i]=c/5,c%=5;}return ret+ret%2*5;}