bignumtest.cpp

// C++ program to find sum of two large numbers.
#include<bits/stdc++.h>
using namespace std;

// Function for finding sum of larger numbers
string findSum(string str1, string str2)
{
	// Before proceeding further, make sure length
	// of str2 is larger.
	if (str1.length() > str2.length())
		swap(str1, str2);

	// Take an empty string for storing result
	string str = "";

	// Calculate length of both string
	int n1 = str1.length(), n2 = str2.length();
	int diff = n2 - n1;

	// Initially take carry zero
	int carry = 0;

	// Traverse from end of both strings
	for (int i=n1-1; i>=0; i--)
	{
		// Do school mathematics, compute sum of
		// current digits and carry
		int sum = ((str1[i]-'0') +
				(str2[i+diff]-'0') +
				carry);
		str.push_back(sum%10 + '0');
		carry = sum/10;
	}

	// Add remaining digits of str2[]
	for (int i=n2-n1-1; i>=0; i--)
	{
		int sum = ((str2[i]-'0')+carry);
		str.push_back(sum%10 + '0');
		carry = sum/10;
	}

	// Add remaining carry
	if (carry)
		str.push_back(carry+'0');

	// reverse resultant string
	reverse(str.begin(), str.end());

	return str;
}

// Driver code
int main()
{
    string str1;
    string str2;
    cin >> str1;
    cin >> str2;
	cout << findSum(str1, str2);
	return 0;
}