【大数模板+十进制+for_loop】Multiply Strings

class Solution {
public:
#define MAXN 200 //最大位数  @@@@error: MAXN as 100 is too little
    //to=axb
    int multi(int *a,int n,int *b,int m,int *to){
        //to should have been set to zero     to=axb
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                to[i+j]+=a[j]*b[i];
            }
        }
        int t=sim(to,m+n-1);
        return t;
    }
    //十进制化s[]
    int sim(int *s,int n){//位数
        int in=0;
        for(int i=0;i<n;i++){
            int c=in+s[i];
            s[i]=c%10,in=c/10;
        }
        int i;
        for(i=n;i<MAXN;i++){////@error for(i=n;i<MAXN;i++) i not declared,should be for(int i)
            if(in<=0) break;
            else s[i]=in%10,in/=10;
        }
        //@@@error:需要去掉头部多余的0
        while(i>0&&s[i]==0) i--;
        return i+1;
    }
    //main函数
    string multiply(string num1,string num2) {
        int a[MAXN],b[MAXN],c[MAXN];
        memset(a,sizeof(a));memset(b,sizeof(a));memset(c,sizeof(a));
        int n=num1.length(),m=num2.length();
        
        for(int i=0;i<num1.size();i++)         
            a[i]=num1[n-1-i]-'0';
        for(int i=0;i<num2.size();i++)         
            b[i]=num2[m-1-i]-'0';
            
        int s=multi(a,n,b,m,c);
        
        string res="";
        for(int i=s-1;i>=0;i--){
            res+='0'+c[i];
        }
        
        return res;
    }
};