Fibonacci Big Number.cpp

#include <iostream>
#include <sstream>

using namespace std;

string itos(int n)
{
	stringstream ss;
	ss << n;
	return ss.str();
}

string strAddStr(string str1, string str2)
{	
	if (str1.size() < str2.size()) {
		string temp = str2;
		str2 = str1;
		str1 = temp;
	}
	int size1 = str1.size();
	int size2 = str2.size();
	
	if (size1 > size2) {
		for (int i = 0; i < size1 - size2; i++) {
			str2 = "0" + str2;
		}
	}
	size1 = size2 = str1.size();
	
	int temp = 0, index;
	string ansStr = "";
	
	for (index = size1-1; index >= 0; index--) {
		int addedNum = ((int)str1[index]-48) + ((int)str2[index]-48);
		if (temp != 0) {
			addedNum = addedNum + temp;
			temp = 0;
		}
		if (addedNum >= 10) {
			temp = addedNum / 10;
			int n = addedNum % 10;
			ansStr = itos(n) + ansStr;
		}
		else {
			ansStr = itos(addedNum) + ansStr;
		}
	}
	
	if (temp != 0) {
		ansStr = itos(temp) + ansStr;
	}
	return ansStr;
}

string fib(int n)
{
	string str1 = "0";
	string str2 = "1";
	string strAns;
	
	if (n == 0 || n == 1)
	{
		return itos(n);
	}
	for (int i = 0; i < n-1; i++) {
		strAns = strAddStr(str1, str2);
		str1 = str2;
		str2 = strAns;
	}
	return strAns;
}

int main(int argc, char *argv[]) {
    int input;
	
    cout << "Please input number for N: ";
	cin >> input;
	string ans = fib(input);
	cout << "ANS: " << ans << endl;
	
	return 0;
}