如:
long long down_diff=c1*d1+c2*d2+....cn*dn;
long long up_diff=0;
diff=0;
min_index=1;
min_diff=0;
int i;
for (i=2;i<=n;i++)
{
up_diff+=c(i-1)*u(i-1);
down_diff-=c(i-1)*d(i-1);
diff+=up_diff-down_diff;
if(diff<min_diff)
{
min_diff=diff;
min_index=i;
}
}
printf("%d\n",min_index);