int n = ch - '0';

string s;

int carry = 0;

int x;

for(int i=num.size()-1; i>=0; i--){

x = (num[i]-'0') * n + carry;

carry = x/10;

s.insert(s.begin(), x%10+'0');

}

if (carry>0) {

s.insert(s.begin(), carry+'0');

}

return s;

string s;

int carry=0;

int x;

int n1 = num1.size();

int n2 = num2.size();

int i, j;

for(i=n1-1, j=n2-1; i>=0 || j>=0; i--, j--){

int x1 = i>=0 ? num1[i]-'0' : 0;

int x2 = j>=0 ? num2[j]-'0' : 0;

x = x1 + x2 + carry;

carry = x/10;

s.insert(s.begin(), x%10+'0');

}

if (carry>0) {

s.insert(s.begin(), carry+'0');

}

return s;

if (num1.size()<=0 || num2.size()<=0) return "";

int shift=0;

string result="0";

for (int i=num1.size()-1; i>=0; i--) {

string s = multiply(num2, num1[i]);

for(int j=0; j<shift; j++){

s.insert(s.end(), '0');

}

result = strPlus(result, s);

shift++;

}

//check if it is zero

if (result[0]=='0') return "0";

return result;

public:

string strPlus(string& num1, string& num2) {

if (num1.size()==0) return num2;

if (num2.size()==0) return num1;

if ( num1.size() < num2.size() ) {

swap(num1, num2);

}

string s;

int carry=0;

int x;

for (int i=num1.size()-1, j=num2.size()-1; i>=0; i--, j--) {

x = num1[i]-'0' + carry;

if(j>=0){

x += num2[j]-'0';

}

s.insert(s.begin(), x%10+'0');

carry = x/10;

}

if (carry>0) {

s.insert(s.begin(), carry+'0');

}

return s;

}

string multiply(string num1, string num2) {

if (num1.size()<=0 || num2.size()<=0) return "";

string result;

for ( int i=num1.size()-1; i>=0; i--) {

int carry = 0;

string val;

for( int j=num2.size()-1; j>=0; j--) {

int v = (num2[j]-'0') * (num1[i]-'0') + carry;

val.insert(val.begin(), v%10+'0');

carry = v/10;

}

if (carry) val.insert(val.begin(), carry+'0');

for (unsigned int j=i; j<num1.size()-1; j++) {

val.push_back('0');

}

result = strPlus(result, val);

}

//check if it is zero

if (result[0]=='0') return "0";

return result;

}

{

string s1="20";

string s2="25";

if (argc>2){

s1 = argv[1];

s2 = argv[2];

}

cout << s1 << " * " << s2 << " = " << multiply(s1, s2) << endl;

return 0;