高精乘低精
cpp
vector<int> mul(vector<int>&A,int b){
vector<int> C;
int t=0;
for(int i=0;i<A.size()||t;i++){
if(i<A.size())t+=A[i]*b;
C.push_back(t%10);
t/=10;
}
while(C.size()>1&&C.back()==0)C.pop_back();
return C;
}