Big Number add and multi

bign.cpp

#include <iostream>
#define OVERFLOW 2
#define ROW b_len
#define COL a_len+b_len+OVERFLOW

using namespace std;

string add (string &s1, string &s2){
	int carry=0,sum,i;

	string  min=s1,
	max=s2,
	result = "";

	if (s1.length()>s2.length()){
		max = s1;
		min = s2;
	} else {
		max = s2;
		min = s1;
	}

	for (i = min.length()-1; i>=0; i--){
		sum = min[i] + max[i + max.length() - min.length()] + carry - 2*'0';

		carry = sum/10;
		sum %=10;

		result = (char)(sum + '0') + result;
	}

	i = max.length() - min.length()-1;

	while (i>=0){
		sum = max[i] + carry - '0';
		carry = sum/10;
		sum%=10;

		result = (char)(sum + '0') + result;
		i--;
	}

	if (carry!=0){
		result = (char)(carry + '0') + result;
	}       

	return result;
}

int getCarry(int num) {
	int carry = 0;
	if(num>=10) {
		while(num!=0) {
			carry = num %10;
			num = num/10;
		}
	} else carry = 0;
	return carry;
}

int num(char a) {
	return int(a)-48;
}

string mult(string a, string b) {
	string ret;
	int a_len = a.length();
	int b_len = b.length();
	int mat[ROW][COL];
	for(int i =0; i<ROW; ++i) {
		for(int j=0; j<COL; ++j) {
			mat[i][j] = 0;
		}
	}

	int carry=0, n,x=a_len-1,y=b_len-1;
	for(int i=0; i<ROW; ++i) {
		x=a_len-1;
		carry = 0;
		for(int j=(COL-1)-i; j>=0; --j) {
			if((x>=0)&&(y>=0))  {
				n = (num(a[x])*num(b[y]))+carry;
				mat[i][j] = n%10;
				carry = getCarry(n);
			} else if((x>=-1)&&(y>=-1)) mat[i][j] = carry;
			x=x-1;
		}
		y=y-1;
	}

	carry = 0;
	int sum_arr[COL];
	for(int i =0; i<COL; ++i) sum_arr[i] = 0;
	for(int i=0; i<ROW; ++i) {
		for(int j=COL-1; j>=0; --j) {
			sum_arr[j] += (mat[i][j]);
		}
	}
	int temp;
	for(int i=COL-1; i>=0; --i) {
		sum_arr[i] += carry;
		temp = sum_arr[i];
		sum_arr[i] = sum_arr[i]%10;
		carry = getCarry(temp);
	}

	for(int i=0; i<COL; ++i) {
		ret.push_back(char(sum_arr[i]+48));
	}

	while(ret[0]=='0'){
		ret = ret.substr(1,ret.length()-1);
	}
	return ret;
}


int main (){
	string a,b;

	cin >> a >> b;

	cout << "Add: " << add(a,b) << endl;
	cout << "Multi: " << mult(a,b) << endl;

	return 0;
}