C++实现大整数乘法（字符串乘法）

#include<iostream>#include<algorithm>#include<string>using namespace std; string add(string a,string b){ if(a.length()==0) return b; if(b.length()==0) return a; a.length()<b.length()?a.swap(b):a.length();  reverse(a.begin(),a.end()); reverse(b.begin(),b.end()); int flag=0; for(int i=0;i<a.length();i++) { int aInt=a[i]-'0'; int bInt=0; if(i<b.length())  bInt=b[i]-'0'; int result=aInt+bInt+flag; a[i]=result%10+'0'; flag=result/10; } if(flag!=0) { a=a+(char)(flag+'0'); } reverse(a.begin(),a.end()); return a;}string multiply(std::string strMultiplierA,char x){ int b=x-'0'; int flag=0; for(int i=strMultiplierA.length()-1;i>=0;i--) { int a=strMultiplierA[i]-'0'; int result=a*b+flag; strMultiplierA[i]=result%10+'0'; flag=result/10; } if(flag!=0) strMultiplierA=(char)(flag+'0')+strMultiplierA; while(strMultiplierA.length()>1&&strMultiplierA[0]=='0') strMultiplierA=strMultiplierA.substr(1,strMultiplierA.length()); return strMultiplierA;} /***************************************************************************** Prototype  : multiply Description : 两个任意长度的长整数相乘, 输出结果 Input Param :         const std::string strMultiplierA 乘数A        const std::string strMultiplierB 乘数B Output    :         std::string strRst      乘法结果 Return Value :         int            0 正确                      -1 异常*****************************************************************************/int multiply (const std::string strMultiplierA,const std::string strMultiplierB, std::string &strRst) {   /* 在这里实现功能 */  if(strMultiplierA.length()<=0||strMultiplierB.length()<=0) return -1; bool flag=false;//false"+" true"-" string strA=strMultiplierA,strB=strMultiplierB; if(strMultiplierA[0]=='-') { flag=~flag; strA=strMultiplierA.substr(1,strMultiplierA.length()); } if(strMultiplierB[0]=='-') { flag==true?flag=false:flag=true; strB=strMultiplierB.substr(1,strMultiplierB.length()); }  for(int i=strB.length()-1;i>=0;i--) { string result=multiply(strA,strB[i]); int j=i; while(++j<strB.length())  result=result+"0"; // while(result.length()>1&&result[0]=='0') // result=result.substr(1,result.length()); strRst=add(strRst,result); } while(strRst.length()>1&&strRst[0]=='0') strRst=strRst.substr(1,strRst.length()); if(flag==true&&strRst!="0") strRst='-'+strRst;  return 0;} int main(){ std::string strResult = "";  multiply("-5489324", "0", strResult);  cout<<strResult<<endl;}