[2016-05-09][51nod][1027 大数乘法]

• 时间:2016-05-09 22:01:19 星期一

• 题目编号:[2016-05-09][51nod][1027 大数乘法]

• 题目大意:给出2个大整数A,B，计算A*B的结果。

• 分析:大数相乘,直接上模板

#include<iostream>
#include<iomanip>
#include<string.h>
using namespace std;
const int dlen = 4;
const int maxn = 9999;
const int maxsize = 1000 + 10;
class bignum{
public:
        int a[500];
        int len;
        bignum(){
                len = 1;
                memset(a,0,sizeof(a));
        }
        bignum operator * (const bignum & t)const{
                bignum ret;
                int i,j,up;
                int tmp ,tmp1;
                for( i = 0 ; i < len ; ++i){
                        up = 0;
                        for( j = 0 ; j < t.len ; ++j){
                                tmp = a[i] * t.a[j] + ret.a[i + j] + up;
                                if(tmp > maxn){
                                        tmp1 = tmp - tmp / (maxn + 1) * (maxn + 1);
                                        up = tmp / (maxn + 1);
                                        ret.a[i + j] = tmp1;
                                }else {
                                        up = 0;
                                        ret.a[i + j] = tmp;
                                }
                        }
                        if(up){
                                ret.a[i + j] = up;
                        }                       
                }
                ret.len = i + j;
                while(ret.a[ret.len - 1] == 0 && ret.len > 1)   --ret.len;
                return ret;
        }
        friend  istream& operator >> (istream & in,bignum &b){
                char ch[maxsize * 4];
                int i = -1;
                in >> ch;
                int l = strlen(ch);
                int count = 0,sum = 0;
                int ed = 0;
                for(int i = l - 1 ; i >= ed ;){
                        sum = 0;
                        int t = 1;
                        for(int j = 0 ; j < 4 && i >= ed ; ++j,--i,t*=10){
                                sum += (ch[i] - '0') * t;
                        }
                        b.a[count++] =sum;
                }
                b.len = count++;
                return in;
        }
        friend ostream & operator << (ostream & out, const bignum &b){
                int i;    
                out<<b.a[b.len - 1];
                for(i = b.len - 2 ; i >=0 ; --i){
                        out<<setw(4)<<setfill('0')<<b.a[i];
                }
                return out;      
        }
};
int main(){
        bignum a,b;
        cin>>a>>b;
        cout<<a*b<<"\n";
        return 0;
}

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