Addition and multiplication of large integers;

Addition and multiplication of large integers ;
1. Add

#include<bits/stdc++.h>
using namespace std;
int a[10000],b[10000],c[10000];
char a1[10000],b1[10000];
int cnt,i,j;
int main(){
    
  cin>>a1>>b1;
  int lena=strlen(a1);
  int lenb=strlen(b1);
  int len=max(lena,lenb)+1;
  for(i=1;i<=lena;i++)a[i]=a1[lena-i]-'0';
  for(i=1;i<=lenb;i++)b[i]=b1[lenb-i]-'0';
  for(i=1;i<=len;i++){
    

    c[i]+=a[i]+b[i];
    c[i+1]=c[i]/10;
    c[i]=c[i]%10;

}
	while(c[len]==0&&len>1)len--;
	for(i=len;i>=1;i--)cout<<c[i];
	return 0;
}

Multiplication of large integers ;

#include<bits/stdc++.h>
using namespace std;
char a1[10001],b1[10001];// This array is defined for ease of input ; 
int a[10001],b[10001],c[10001];
int i,len,j; 
int main ()
{
    
    cin>>a1>>b1;//
    int lena=strlen(a1);//
	int lenb=strlen(b1);//
    for(i=1;i<=lena;i++)a[i]=a1[lena-i]-'0';// Reverse input ; 
    for(i=1;i<=lenb;i++)b[i]=b1[lenb-i]-'0';// Reverse input ; 
	for(i=1;i<=lenb;i++)
	for(j=1;j<=lena;j++)
	c[i+j-1]+=a[j]*b[i];// When calculating the multiplication, do not consider the carry first , Then add it up .
    for(i=1;i<lena+lenb;i++)//j It refers to the normal calculation carry （ Multiply bit by bit ）, and i It is what we call dislocation addition . 
	  if(c[i]>9)// Then consider carry ; 
	{
    
		c[i+1]+=c[i]/10;
		c[i]%=10;
	}
	len=lena+lenb;
    while(c[len]==0&&len>1)len--;// Remove leading zeros  ; 
    for(i=len;i>=1;i--)cout<<c[i];// Output in reverse order ; 
    return 0;
}