Resta de dos números grandes usando complemento a 9

C++

// C++ Program to implement
// the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to return sum of two
// large numbers given as strings
string sumBig(string a, string b)
{
 
    // Compare their lengths
    if (a.length() > b.length())
        swap(a, b);
 
    // Stores the result
    string str = "";
 
    // Store the respective lengths
    int n1 = a.length(),
        n2 = b.length();
 
    int diff = n2 - n1;
 
    // Initialize carry
    int carry = 0;
 
    // Traverse from end of both strings
    for (int i = n1 - 1; i >= 0; i--) {
        // Compute sum of
        // current digits and carry
        int sum = ((a[i] - '0')
                   + (b[i + diff] - '0')
                   + carry);
 
        // Store the result
        str.push_back(sum % 10 + '0');
 
        // Update carry
        carry = sum / 10;
    }
 
    // Add remaining digits of str2[]
    for (int i = n2 - n1 - 1; i >= 0; i--) {
 
        int sum = ((b[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;
}
 
// Function return 9's
// complement of given number
string complement9(string v)
{
    // Stores the complement
    string complement = "";
 
    for (int i = 0; i < v.size(); i++) {
 
        // Subtract every bit from 9
        complement += '9' - v[i] + '0';
    }
 
    // Return the result
    return complement;
}
 
// Function returns subtraction
// of two given numbers as strings
string subtract(string a, string b)
{
 
    // If second string is larger
    if (a.length() < b.length())
        swap(a, b);
 
    // Calculate respective lengths
    int l1 = a.length(),
        l2 = b.length();
 
    // If lengths are equal
    int diffLen = l1 - l2;
 
    for (int i = 0; i < diffLen; i++) {
 
        // Insert 0's to the beginning
        // of b to make both the lengths equal
        b = "0" + b;
    }
 
    // Add (complement of B) and A
    string c = sumBig(a, complement9(b));
 
    // If length of new string is greater
    // than length of first string,
    // than carry is generated
    if (c.length() > a.length()) {
        string::iterator it;
 
        // bit1 is the carry bit
        char bit1 = c[0];
        string bit = { bit1 };
        it = c.begin();
        c.erase(it);
        c = sumBig(c, bit);
        return c;
    }
 
    // If both lengths are equal
    else {
        return complement9(c);
    }
}
 
// Driver Code
int main()
{
 
    string str1 = "12345678987654321";
    string str2 = "22324324343";
 
    cout << subtract(str1, str2) << endl;
 
    return 0;
}