大数相乘.cpp

#include<iostream>
#include<stdlib.h> 
#include <string.h>
#include<stdio.h>
using namespace std;

string calculate(string a,string b);
int resultList[256];

int main(){
	string fn,sn;
	cout<<"输入第一个数字和第二个数字.\n"; 
	cin>>fn>>sn;
	string result;
	result=calculate(fn,sn);
	cout<<result<<endl;
}

string calculate(string a,string b){
	int temp=0,i=0,j=0,sumOfZero=0,carryBit=0,fn,sn,min,k=0,moveStep=0,count=-1;
	string flag,zeros,result;
	
	if(a.length() > b.length()) flag = "b";//找出a,b字符串中较短的那个. 
	else flag = "a";
	
	if(flag=="a"){         //若a较短,则在a前面补0,如a="123"->a="000123"; 
		min = a.length();
		sumOfZero = b.length() - a.length();
		for(i=0;i<sumOfZero;i++){
			zeros+="0";
		}
		a= zeros + a;
	}
	else{
		min = b.length();
		sumOfZero = a.length() - b.length();
		for(i=0;i<sumOfZero;i++){
			zeros+="0";
		}
		b= zeros + b;
	} 

	for(i=(a.length()-1);i>=0;i--){
		sn = b[i]-'0';                            
		for(j=(a.length()-1);j>=0;j--){
			fn = a[j]-'0';
			temp = fn*sn+carryBit;
			carryBit = 0;
			if(temp>9){
				carryBit = temp/10;
				temp=temp%10;
			}
			resultList[j-k+min] = resultList[j-k+min] + temp;
		}
		if(carryBit!=0) {
			moveStep++;
			resultList[j-k+min] = resultList[j-1-k+min] + carryBit;
		}
		carryBit=0;
		k++;
	} 
	
	for(i=(a.length()+min)-1;i>=(min-moveStep);i--){
		if(resultList[i]>9){
			carryBit = resultList[i]/10;
			resultList[i] = resultList[i]%10;
			resultList[i-1] = resultList[i-1]+carryBit;
			carryBit = 0;
		}
	}
	for(j=0;j<a.length()+min;j++){
		if(resultList[i]!=0){
			count++;
			break;
		}
	}
	for(i=0;i<a.length()+min;i++) 
	{	
		
		if(resultList[i]!=0 && i!=count){
			char temps[256];
			temp = resultList[i];
			itoa(temp,temps,10);
			result+=temps;
		}
	}
	return result;
}