Difference of two large numbers

Given two numbers as strings. The numbers may be very large (may not fit in long long int), the task is to find the difference between these two numbers.

Examples:

Input : str1 = "11443333311111111100", 
        str2 =     "1144422222221111"
Output : 11442188888888889989

Input :str1 = "122387876566565674",
       str2 =     "31435454654554"
Output : 122356441111911120

This is simply based on school mathematics. We traverse both strings from the end, one by one subtract digits. 

1. Reverse both strings.
2. Keep subtracting digits one by one from 0’th index (in reversed strings) to the end of a smaller string, append the diff if it’s positive to end of the result. If difference(diff) is negative then add 10 and keep track of carry as 1 if it’s positive then carry is 0.
3. Finally, reverse the result.
   

Below is the implementation of the above idea: 

C++

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

// Returns true if str1 is smaller than str2.
bool isSmaller(string str1, string str2)
{
    // Calculate lengths of both string
    int n1 = str1.length(), n2 = str2.length();

    if (n1 < n2)
        return true;
    if (n2 < n1)
        return false;

    for (int i = 0; i < n1; i++)
        if (str1[i] < str2[i])
            return true;
        else if (str1[i] > str2[i])
            return false;

    return false;
}

// Function for find difference of larger numbers
string findDiff(string str1, string str2)
{
    // Before proceeding further, make sure str1
    // is not smaller
    if (isSmaller(str1, str2))
        swap(str1, str2);

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

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

    // Reverse both of strings
    reverse(str1.begin(), str1.end());
    reverse(str2.begin(), str2.end());

    int carry = 0;

    // Run loop till small string length
    // and subtract digit of str1 to str2
    for (int i = 0; i < n2; i++) {
        // Do school mathematics, compute difference of
        // current digits

        int sub
            = ((str1[i] - '0') - (str2[i] - '0') - carry);

        // If subtraction is less than zero
        // we add then we add 10 into sub and
        // take carry as 1 for calculating next step
        if (sub < 0) {
            sub = sub + 10;
            carry = 1;
        }
        else
            carry = 0;

        str.push_back(sub + '0');
    }

    // subtract remaining digits of larger number
    for (int i = n2; i < n1; i++) {
        int sub = ((str1[i] - '0') - carry);

        // if the sub value is -ve, then make it positive
        if (sub < 0) {
            sub = sub + 10;
            carry = 1;
        }
        else
            carry = 0;

        str.push_back(sub + '0');
    }

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

    return str;
}

// Driver code
int main()
{
    string str1 = "978";
    string str2 = "12977";

    // Function call
    cout << findDiff(str1, str2) << endl;

    string s1 = "100";
    string s2 = "1000000";

    // Function call
    cout << findDiff(s1, s2);

    return 0;
}

Java

// Java program to find difference of two large numbers.
import java.util.*;

class GFG {

    // Returns true if str1 is smaller than str2.
    static boolean isSmaller(String str1, String str2)
    {
        // Calculate lengths of both string
        int n1 = str1.length(), n2 = str2.length();
        if (n1 < n2)
            return true;
        if (n2 < n1)
            return false;

        for (int i = 0; i < n1; i++)
            if (str1.charAt(i) < str2.charAt(i))
                return true;
            else if (str1.charAt(i) > str2.charAt(i))
                return false;

        return false;
    }

    // Function for find difference of larger numbers
    static String findDiff(String str1, String str2)
    {
        // Before proceeding further, make sure str1
        // is not smaller
        if (isSmaller(str1, str2)) {
            String t = str1;
            str1 = str2;
            str2 = t;
        }

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

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

        // Reverse both of strings
        str1 = new StringBuilder(str1).reverse().toString();
        str2 = new StringBuilder(str2).reverse().toString();

        int carry = 0;

        // Run loop till small string length
        // and subtract digit of str1 to str2
        for (int i = 0; i < n2; i++) {
            // Do school mathematics, compute difference of
            // current digits
            int sub
                = ((int)(str1.charAt(i) - '0')
                   - (int)(str2.charAt(i) - '0') - carry);

            // If subtraction is less than zero
            // we add then we add 10 into sub and
            // take carry as 1 for calculating next step
            if (sub < 0) {
                sub = sub + 10;
                carry = 1;
            }
            else
                carry = 0;

            str += (char)(sub + '0');
        }

        // subtract remaining digits of larger number
        for (int i = n2; i < n1; i++) {
            int sub = ((int)(str1.charAt(i) - '0') - carry);

            // if the sub value is -ve, then make it
            // positive
            if (sub < 0) {
                sub = sub + 10;
                carry = 1;
            }
            else
                carry = 0;

            str += (char)(sub + '0');
        }

        // reverse resultant string
        return new StringBuilder(str).reverse().toString();
    }

    // Driver code
    public static void main(String[] args)
    {
        String str1 = "978";
        String str2 = "12977";

        // Function call
        System.out.println(findDiff(str1, str2));

        String s1 = "100";
        String s2 = "1000000";

        // Function call
        System.out.println(findDiff(s1, s2));
    }
}

// This code is contributed by mits

Python3

# Python 3 program to find difference of two large numbers.

# Returns true if str1 is smaller than str2.


def isSmaller(str1, str2):

    # Calculate lengths of both string
    n1 = len(str1)
    n2 = len(str2)

    if (n1 < n2):
        return True
    if (n2 < n1):
        return False

    for i in range(n1):
        if (str1[i] < str2[i]):
            return True
        elif (str1[i] > str2[i]):
            return False

    return False

# Function for find difference of larger numbers


def findDiff(str1, str2):

    # Before proceeding further, make sure str1
    # is not smaller
    if (isSmaller(str1, str2)):
        temp = str1
        str1 = str2
        str2 = temp

    # Take an empty string for storing result
    str3 = ""

    # Calculate length of both string
    n1 = len(str1)
    n2 = len(str2)

    # Reverse both of strings
    str1 = str1[::-1]
    str2 = str2[::-1]

    carry = 0

    # Run loop till small string length
    # and subtract digit of str1 to str2
    for i in range(n2):

        # Do school mathematics, compute difference of
        # current digits

        sub = ((ord(str1[i])-ord('0'))-(ord(str2[i])-ord('0'))-carry)

        # If subtraction is less than zero
        # we add then we add 10 into sub and
        # take carry as 1 for calculating next step
        if (sub < 0):

            sub = sub + 10
            carry = 1

        else:
            carry = 0

        str3 = str3+str(sub)

    # subtract remaining digits of larger number
    for i in range(n2, n1):

        sub = ((ord(str1[i])-ord('0')) - carry)

        # if the sub value is -ve, then make it positive
        if (sub < 0):

            sub = sub + 10
            carry = 1

        else:
            carry = 0

        str3 = str3+str(sub)

    # reverse resultant string
    str3 = str3[::-1]

    return str3


# Driver code
if __name__ == "__main__":
    str1 = "978"
    str2 = "12977"

    # Function call
    print(findDiff(str1, str2))

    s1 = "100"
    s2 = "1000000"

    # Function call
    print(findDiff(s1, s2))

# This code is contributed by ChitraNayal

C#

// C# program to find difference of two large numbers.
using System;
using System.Collections;

class GFG {

    // Returns true if str1 is smaller than str2.
    static bool isSmaller(string str1, string str2)
    {
        // Calculate lengths of both string
        int n1 = str1.Length, n2 = str2.Length;
        if (n1 < n2)
            return true;
        if (n2 < n1)
            return false;

        for (int i = 0; i < n1; i++)
            if (str1[i] < str2[i])
                return true;
            else if (str1[i] > str2[i])
                return false;

        return false;
    }

    // Function for find difference of larger numbers
    static string findDiff(string str1, string str2)
    {
        // Before proceeding further, make sure str1
        // is not smaller
        if (isSmaller(str1, str2)) {
            string t = str1;
            str1 = str2;
            str2 = t;
        }

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

        // Calculate length of both string
        int n1 = str1.Length, n2 = str2.Length;

        // Reverse both of strings
        char[] ch1 = str1.ToCharArray();
        Array.Reverse(ch1);
        str1 = new string(ch1);
        char[] ch2 = str2.ToCharArray();
        Array.Reverse(ch2);
        str2 = new string(ch2);

        int carry = 0;

        // Run loop till small string length
        // and subtract digit of str1 to str2
        for (int i = 0; i < n2; i++) {
            // Do school mathematics, compute difference of
            // current digits
            int sub = ((int)(str1[i] - '0')
                       - (int)(str2[i] - '0') - carry);

            // If subtraction is less than zero
            // we add then we add 10 into sub and
            // take carry as 1 for calculating next step
            if (sub < 0) {
                sub = sub + 10;
                carry = 1;
            }
            else
                carry = 0;

            str += (char)(sub + '0');
        }

        // subtract remaining digits of larger number
        for (int i = n2; i < n1; i++) {
            int sub = ((int)(str1[i] - '0') - carry);

            // if the sub value is -ve, then make it
            // positive
            if (sub < 0) {
                sub = sub + 10;
                carry = 1;
            }
            else
                carry = 0;

            str += (char)(sub + '0');
        }

        // reverse resultant string
        char[] ch3 = str.ToCharArray();
        Array.Reverse(ch3);
        return new string(ch3);
    }

    // Driver code
    public static void Main()
    {
        string str1 = "978";
        string str2 = "12977";

        // Function call
        Console.WriteLine(findDiff(str1, str2));

        string s1 = "100";
        string s2 = "1000000";

        // Function call
        Console.WriteLine(findDiff(s1, s2));
    }
}

// This code is contributed by mits

PHP

<?php
// PHP program to find difference of two large numbers.

// Returns true if str1 is smaller than str2.
function isSmaller($str1, $str2)
{
    // Calculate lengths of both string
    $n1 = strlen($str1);
    $n2 = strlen($str2);

    if ($n1 < $n2)
    return true;
    if ($n2 < $n1)
    return false;

    for ($i=0; $i<$n1; $i++)
    if ($str1[$i] < $str2[$i])
        return true;
    else if ($str1[$i] > $str2[$i])
        return false;

    return false;
}

// Function for find difference of larger numbers
function findDiff($str1, $str2)
{
    // Before proceeding further, make sure str1
    // is not smaller
    if (isSmaller($str1, $str2)){
        $t=$str1;
        $str1=$str2;
        $str2=$t;
    }

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

    // Calculate length of both string
    $n1 = strlen($str1);
    $n2 = strlen($str2);

    // Reverse both of strings
    $str1=strrev($str1);
    $str2=strrev($str2);

    $carry = 0;

    // Run loop till small string length
    // and subtract digit of str1 to str2
    for ($i=0; $i<$n2; $i++)
    {
        // Do school mathematics, compute difference of
        // current digits

        $sub=((ord($str1[$i])-ord('0'))-(ord($str2[$i])-ord('0'))-$carry);

        // If subtraction is less than zero
        // we add then we add 10 into sub and
        // take carry as 1 for calculating next step
        if ($sub < 0)
        {
            $sub = $sub + 10;
            $carry = 1;
        }
        else
            $carry = 0;

        $str.=chr($sub+48);
    }

    // subtract remaining digits of larger number
    for ($i=$n2; $i<$n1; $i++)
    {
        $sub = ((ord($str1[$i])-ord('0')) - $carry);

        // if the sub value is -ve, then make it positive
        if ($sub < 0)
        {
            $sub = $sub + 10;
            $carry = 1;
        }
        else
            $carry = 0;

        $str.=chr($sub+48);
    }

    // reverse resultant string
    $str=strrev($str);

    return $str;
}

// Driver code

    $str1 = "978";
    $str2 = "12977";

    // Function call
    echo findDiff($str1, $str2)."\n";

    $s1 = "100";
    $s2 = "1000000";

    // Function call
    echo findDiff($s1,$s2);

// This code is contributed by mits
?>

Javascript

<script>

// Javascript program to find difference
// of two large numbers.

    // Returns true if str1 is smaller than str2.
    function isSmaller(str1,str2)
    {
        // Calculate lengths of both string
        let n1 = str1.length, n2 = str2.length;
        if (n1 < n2)
            return true;
        if (n2 < n1)
            return false;

        for (let i = 0; i < n1; i++)
            if (str1[i] < str2[i])
                return true;
            else if (str1[i] > str2[i])
                return false;

        return false;
    }

     // Function for find difference
     // of larger numbers
    function findDiff(str1,str2)
    {
        // Before proceeding further,
        // make sure str1
        // is not smaller
        if (isSmaller(str1, str2)) {
            let t = str1;
            str1 = str2;
            str2 = t;
        }

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

        // Calculate length of both string
        let n1 = str1.length, n2 = str2.length;

        // Reverse both of strings
        str1 = str1.split("").reverse().join("")
        str2 = str2.split("").reverse().join("")

        let carry = 0;

        // Run loop till small string length
        // and subtract digit of str1 to str2
        for (let i = 0; i < n2; i++)
        {
            // Do school mathematics,
            // compute difference of
            // current digits
            let sub
                = ((str1[i].charCodeAt(0) -
                '0'.charCodeAt(0))
                   - (str2[i].charCodeAt(0) -
                   '0'.charCodeAt(0)) - carry);

            // If subtraction is less than zero
            // we add then we add 10 into sub and
            // take carry as 1 for calculating next step
            if (sub < 0) {
                sub = sub + 10;
                carry = 1;
            }
            else
                carry = 0;

            str += String.fromCharCode(sub +
            '0'.charCodeAt(0));
        }

        // subtract remaining digits of larger number
        for (let i = n2; i < n1; i++) {
            let sub = ((str1[i].charCodeAt(0) -
            '0'.charCodeAt(0)) - carry);

            // if the sub value is -ve, then make it
            // positive
            if (sub < 0) {
                sub = sub + 10;
                carry = 1;
            }
            else
                carry = 0;

            str += String.fromCharCode(sub +
            '0'.charCodeAt(0));
        }

        // reverse resultant string
        return str.split("").reverse().join("")
    }

    // Driver code

    let str1 = "978";
    let str2 = "12977";

    document.write(findDiff(str1, str2)+"<br>");

    let s1 = "100";
    let s2 = "1000000";
    document.write(findDiff(s1, s2)+"<br>");

    // This code is contributed by rag2127

</script>

Output

11999
0999900

Time Complexity: O(n1 + n2)

Auxiliary Space: O(max(n1, n2))
Optimized solution :
We can avoid the first two string reverse operations by traversing them from the end. 

Below is the optimized solution. 

C++

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

// Returns true if str1 is smaller than str2,
// else false.
bool isSmaller(string str1, string str2)
{
    // Calculate lengths of both string
    int n1 = str1.length(), n2 = str2.length();

    if (n1 < n2)
        return true;
    if (n2 < n1)
        return false;

    for (int i = 0; i < n1; i++) {
        if (str1[i] < str2[i])
            return true;
        else if (str1[i] > str2[i])
            return false;
    }
    return false;
}

// Function for finding difference of larger numbers
string findDiff(string str1, string str2)
{
    // Before proceeding further, make sure str1
    // is not smaller
    if (isSmaller(str1, str2))
        swap(str1, str2);

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

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

    // Initially take carry zero
    int carry = 0;

    // Traverse from end of both strings
    for (int i = n2 - 1; i >= 0; i--) {
        // Do school mathematics, compute difference of
        // current digits and carry
        int sub = ((str1[i + diff] - '0') - (str2[i] - '0')
                   - carry);
        if (sub < 0) {
            sub = sub + 10;
            carry = 1;
        }
        else
            carry = 0;

        str.push_back(sub + '0');
    }

    // subtract remaining digits of str1[]
    for (int i = n1 - n2 - 1; i >= 0; i--) {
        if (str1[i] == '0' && carry) {
            str.push_back('9');
            continue;
        }
        int sub = ((str1[i] - '0') - carry);
        if (i > 0 || sub > 0) // remove preceding 0's
            str.push_back(sub + '0');
        carry = 0;
    }

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

    return str;
}

// Driver code
int main()
{
    string str1 = "88";
    string str2 = "1079";

    // Function call
    cout << findDiff(str1, str2);
    return 0;
}

Java

// Java program to find difference of two large numbers.

class GFG {

    // Returns true if str1 is smaller than str2,
    // else false.
    static boolean isSmaller(String str1, String str2)
    {
        // Calculate lengths of both string
        int n1 = str1.length(), n2 = str2.length();

        if (n1 < n2)
            return true;
        if (n2 < n1)
            return false;

        for (int i = 0; i < n1; i++) {
            if (str1.charAt(i) < str2.charAt(i))
                return true;
            else if (str1.charAt(i) > str2.charAt(i))
                return false;
        }
        return false;
    }

    // Function for finding difference of larger numbers
    static String findDiff(String str1, String str2)
    {
        // Before proceeding further, make sure str1
        // is not smaller
        if (isSmaller(str1, str2)) {
            String t = str1;
            str1 = str2;
            str2 = t;
        }

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

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

        // Initially take carry zero
        int carry = 0;

        // Traverse from end of both strings
        for (int i = n2 - 1; i >= 0; i--) {
            // Do school mathematics, compute difference of
            // current digits and carry
            int sub
                = (((int)str1.charAt(i + diff) - (int)'0')
                   - ((int)str2.charAt(i) - (int)'0')
                   - carry);
            if (sub < 0) {
                sub = sub + 10;
                carry = 1;
            }
            else
                carry = 0;

            str += String.valueOf(sub);
        }

        // subtract remaining digits of str1[]
        for (int i = n1 - n2 - 1; i >= 0; i--) {
            if (str1.charAt(i) == '0' && carry > 0) {
                str += "9";
                continue;
            }
            int sub = (((int)str1.charAt(i) - (int)'0')
                       - carry);
            if (i > 0 || sub > 0) // remove preceding 0's
                str += String.valueOf(sub);
            carry = 0;
        }

        // reverse resultant string
        return new StringBuilder(str).reverse().toString();
    }

    // Driver code
    public static void main(String[] args)
    {
        String str1 = "88";
        String str2 = "1079";

        // Function call
        System.out.println(findDiff(str1, str2));
    }
}

// This code is contributed by mits

Python3

# Python3 program to find difference
# of two large numbers.

# Returns true if str1 is smaller than
# str2, else false.
def isSmaller(str1, str2):

    # Calculate lengths of both string
    n1 = len(str1)
    n2 = len(str2)

    if (n1 < n2):
        return True
    if (n2 < n1):
        return False

    for i in range(n1):
        if (str1[i] < str2[i]):
            return True
        elif (str1[i] > str2[i]):
            return False

    return False

# Function for finding difference
# of larger numbers
def findDiff(str1, str2):

    # Before proceeding further,
    # make sure str1 is not smaller
    if (isSmaller(str1, str2)):
        str1, str2 = str2, str1

    # Take an empty string for
    # storing result
    Str = ""

    # Calculate lengths of both string
    n1 = len(str1)
    n2 = len(str2)
    diff = n1 - n2

    # Initially take carry zero
    carry = 0

    # Traverse from end of both strings
    for i in range(n2 - 1, -1, -1):

        # Do school mathematics, compute
        # difference of current digits
        # and carry
        sub = ((ord(str1[i + diff]) - ord('0')) -
               (ord(str2[i]) - ord('0')) - carry)

        if (sub < 0):
            sub += 10
            carry = 1
        else:
            carry = 0

        Str += chr(sub + ord('0'))

    # Subtract remaining digits of str1[]
    for i in range(n1 - n2 - 1, -1, -1):
        if (str1[i] == '0' and carry):
            Str += '9'
            continue

        sub = (ord(str1[i]) - ord('0')) - carry

        # Remove preceding 0's
        if (i > 0 or sub > 0):
            Str += chr(sub + ord('0'))

        carry = 0

    # Reverse resultant string
    Str = Str[::-1]

    return Str

# Driver code
str1 = "88"
str2 = "1079"

# Function call
print(findDiff(str1, str2))

# This code is contributed by avanitrachhadiya2155

C#

// C# program to find difference of
// two large numbers.
using System;

class GFG {

    // Returns true if str1 is smaller
    // than str2, else false.
    static bool isSmaller(string str1, string str2)
    {
        // Calculate lengths of both string
        int n1 = str1.Length, n2 = str2.Length;

        if (n1 < n2)
            return true;
        if (n2 < n1)
            return false;

        for (int i = 0; i < n1; i++) {
            if (str1[i] < str2[i])
                return true;
            else if (str1[i] > str2[i])
                return false;
        }
        return false;
    }

    // Function for finding difference of
    // larger numbers
    static string findDiff(string str1, string str2)
    {
        // Before proceeding further,
        // make sure str1 is not smaller
        if (isSmaller(str1, str2)) {
            string t = str1;
            str1 = str2;
            str2 = t;
        }

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

        // Calculate lengths of both string
        int n1 = str1.Length, n2 = str2.Length;
        int diff = n1 - n2;

        // Initially take carry zero
        int carry = 0;

        // Traverse from end of both strings
        for (int i = n2 - 1; i >= 0; i--) {
            // Do school mathematics, compute
            // difference of current digits and carry
            int sub = (((int)str1[i + diff] - (int)'0')
                       - ((int)str2[i] - (int)'0') - carry);
            if (sub < 0) {
                sub = sub + 10;
                carry = 1;
            }
            else
                carry = 0;

            str += sub.ToString();
        }

        // subtract remaining digits of str1[]
        for (int i = n1 - n2 - 1; i >= 0; i--) {
            if (str1[i] == '0' && carry > 0) {
                str += "9";
                continue;
            }
            int sub = (((int)str1[i] - (int)'0') - carry);
            if (i > 0 || sub > 0) // remove preceding 0's
                str += sub.ToString();
            carry = 0;
        }

        // reverse resultant string
        char[] aa = str.ToCharArray();
        Array.Reverse(aa);
        return new string(aa);
    }

    // Driver code
    public static void Main()
    {
        String str1 = "88";
        String str2 = "1079";

        // Function call
        Console.WriteLine(findDiff(str1, str2));
    }
}

// This code is contributed by mits

PHP

<?php
// PHP program to find difference of two
// large numbers.

// Returns true if str1 is smaller than
// str2, else false.
function isSmaller($str1, $str2)
{
    // Calculate lengths of both string
    $n1 = strlen($str1);
    $n2 = strlen($str2);

    if ($n1 < $n2)
        return true;
    if ($n2 < $n1)
        return false;

    for ($i = 0; $i < $n1; $i++)
    {
        if ($str1[$i] < $str2[$i])
            return true;
        else if ($str1[$i] > $str2[$i])
            return false;
    }
    return false;
}

// Function for finding difference
// of larger numbers
function findDiff($str1, $str2)
{
    // Before proceeding further, make
    // sure str1 is not smaller
    if (isSmaller($str1, $str2))
    {
        $t = $str1;
        $str1 = $str2;
        $str2 = $t;
    }

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

    // Calculate lengths of both string
    $n1 = strlen($str1);
    $n2 = strlen($str2);
    $diff = $n1 - $n2;

    // Initially take carry zero
    $carry = 0;

    // Traverse from end of both strings
    for ($i = $n2 - 1; $i >= 0; $i--)
    {
        // Do school mathematics, compute
        // difference of current digits and carry
        $sub = ((ord($str1[$i + $diff]) - ord('0')) -
                (ord($str2[$i]) - ord('0')) - $carry);
        if ($sub < 0)
        {
            $sub = $sub + 10;
            $carry = 1;
        }
        else
            $carry = 0;

        $str.=chr($sub + ord("0"));
    }

    // subtract remaining digits of str1[]
    for ($i = $n1 - $n2 - 1; $i >= 0; $i--)
    {
        if ($str1[$i] == '0' && $carry > 0)
        {
            $str.="9";
            continue;
        }
        $sub = (ord($str1[$i]) - ord('0') - $carry);
        if ($i > 0 || $sub > 0) // remove preceding 0's
            $str.=chr($sub + ord("0"));
        $carry = 0;

    }

    // reverse resultant string
    return strrev($str);

}

// Driver code
$str1 = "88";
$str2 = "1079";

// Function call
print(findDiff($str1, $str2));

// This code is contributed by chandan_jnu
?>

Javascript

<script>

    // JavaScript program to find difference
    // of two large numbers.

    // Returns true if str1 is smaller
    // than str2, else false.
    function isSmaller(str1, str2)
    {
        // Calculate lengths of both string
        let n1 = str1.length, n2 = str2.length;

        if (n1 < n2)
            return true;
        if (n2 < n1)
            return false;

        for (let i = 0; i < n1; i++) {
            if (str1[i].charCodeAt() < str2[i].charCodeAt())
                return true;
            else if (str1[i].charCodeAt() > str2[i].charCodeAt())
                return false;
        }
        return false;
    }

    // Function for finding difference of
    // larger numbers
    function findDiff(str1, str2)
    {
        // Before proceeding further,
        // make sure str1 is not smaller
        if (isSmaller(str1, str2)) {
            let t = str1;
            str1 = str2;
            str2 = t;
        }

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

        // Calculate lengths of both string
        let n1 = str1.length, n2 = str2.length;
        let diff = n1 - n2;

        // Initially take carry zero
        let carry = 0;

        // Traverse from end of both strings
        for (let i = n2 - 1; i >= 0; i--) {
            // Do school mathematics, compute
            // difference of current digits and carry
            let sub = ((str1[i + diff].charCodeAt() -
                       '0'.charCodeAt())
                   - (str2[i].charCodeAt() -
                     '0'.charCodeAt()) - carry);
            if (sub < 0) {
                sub = sub + 10;
                carry = 1;
            }
            else
                carry = 0;

            str += sub.toString();
        }

        // subtract remaining digits of str1[]
        for (let i = n1 - n2 - 1; i >= 0; i--) {
            if (str1[i] == '0' && carry > 0) {
                str += "9";
                continue;
            }
            let sub = ((str1[i].charCodeAt() -
                       '0'.charCodeAt()) - carry);
            if (i > 0 || sub > 0) // remove preceding 0's
                str += sub.toString();
            carry = 0;
        }

        // reverse resultant string
        let aa = str.split('');
        aa.reverse();
        return aa.join("");
    }

    let str1 = "88";
    let str2 = "1079";

    // Function call
    document.write(findDiff(str1, str2));

</script>

Output

991

Time complexity : O(n1 + n2)

Auxiliary Space: O(max(n1, n2))

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