Sum of all prefixes of given numeric string

Given string str having N characters representing an integer, the task is to calculate the sum of all possible prefixes of the given string.

Example:

Input: str = “1225”
Output: 1360
Explanation: The prefixes of the given string are 1, 12, 122, and 1225 and their sum will be 1 + 12 + 122 + 1225 = 1360.

Input: str = “20”
Output: 22

Approach: The given problem is an implementation-based problem and can be solved by iterating over all the prefixes of the string and maintaining their sum in a string. The sum of two integers represented as strings can be done using the approach discussed here.

Below is the implementation of the above approach:

C++

// C++ program of the above approach
#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);

    // Stores resulting sum
    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;
    for (int i = 0; i < n1; i++) {
        // Compute sum of current
        // digits and carry
        int sum
            = ((str1[i] - '0')
               + (str2[i] - '0')
               + carry);
        str.push_back(sum % 10 + '0');

        // Carry for next step
        carry = sum / 10;
    }

    // Add remaining digits
    for (int i = n1; i < n2; 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 string
    reverse(str.begin(), str.end());

    // Return Answer
    return str;
}

// Function to find sum of all prefixes
// of a string representing a number
string sumPrefix(string str)
{
    // Stores the desired sum
    string sum = "0";

    // Stores the current prefix
    string curPre = "";

    // Loop to iterate str
    for (int i = 0; i < str.length(); i++) {
        // Update current prefix
        curPre += str[i];

        // Update Sum
        sum = findSum(curPre, sum);
    }

    // Return Answer
    return sum;
}

// Driver Code
int main()
{
    string str = "1225";
    cout << sumPrefix(str);

    return 0;
}

Java

// Java program of the above approach
import java.util.*;
class GFG{

// Function for finding sum of larger numbers
static String findSum(String str1, String str2)
{

    // Before proceeding further, make
    // sure length of str2 is larger
    if (str1.length() > str2.length()) {
        String s = str1;
        str1=str2;
        str2=s;
    }

    // Stores resulting sum
    String str = "";

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

    // Reverse both of Strings
    str1 = reverse(str1);
    str2 = reverse(str2);

    int carry = 0;
    for (int i = 0; i < n1; i++) {
        // Compute sum of current
        // digits and carry
        int sum
            = ((str1.charAt(i) - '0')
               + (str2.charAt(i) - '0')
               + carry);
        str+=(char)((sum % 10 + '0'));

        // Carry for next step
        carry = sum / 10;
    }

    // Add remaining digits
    for (int i = n1; i < n2; i++) {
        int sum = ((str2.charAt(i) - '0') + carry);
        str+=(char)(sum % 10 + '0');
        carry = sum / 10;
    }

    // Add remaining carry
    if (carry > 0)
        str += (carry + '0');

    // Reverse String
    str = reverse(str);

    // Return Answer
    return str;
}

// Function to find sum of all prefixes
// of a String representing a number
static String sumPrefix(String str)
{

    // Stores the desired sum
    String sum = "0";

    // Stores the current prefix
    String curPre = "";

    // Loop to iterate str
    for (int i = 0; i < str.length(); i++)
    {

        // Update current prefix
        curPre += (char)(str.charAt(i));

        // Update Sum
        sum = findSum(curPre, sum);
    }

    // Return Answer
    return sum;
}
static String reverse(String input) {
    char[] a = input.toCharArray();
    int l, r = a.length - 1;
    for (l = 0; l < r; l++, r--) {
        char temp = a[l];
        a[l] = a[r];
        a[r] = temp;
    }
    return String.valueOf(a);
}

// Driver Code
public static void main(String[] args)
{
    String str = "1225";
    System.out.print(sumPrefix(str));
}
}

// This code is contributed by shikhasingrajput

Python3

# Python code for the above approach
# Function for finding sum of larger numbers
def findSum(str1, str2):

    # Before proceeding further, make
    # sure length of str2 is larger
    if (len(str1) > len(str2)):
        temp = str1;
        str1 = str2;
        str2 = temp;

    # Stores resulting sum
    str = [];

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

    # Reverse both of strings
    str1 = list(str1)
    str2 = list(str2)
    str1.reverse();
    str2.reverse();

    carry = 0;
    for i in range(n1):

        # Compute sum of current
        # digits and carry
        sum = ((ord(str1[i]) - ord('0')) + (ord(str2[i]) - ord('0')) + carry);
        str.append(chr(sum % 10 + ord('0')));

        # Carry for next step
        carry = sum // 10

    # Add remaining digits
    for i in range(n1, n2):
        sum = ((ord(str2[i]) - ord('0')) + carry);
        str.append(chr(sum % 10 + ord('0')));
        carry = sum // 10

    # Add remaining carry
    if (carry):
        str.append(chr(carry + ord('0')));

    # Reverse string
    str.reverse();

    # Return Answer
    return ''.join(str)

# Function to find sum of all prefixes
# of a string representing a number
def sumPrefix(str):

    # Stores the desired sum
    sum = "0";

    # Stores the current prefix
    curPre = "";

    # Loop to iterate str
    for i in range(len(str)):
        # Update current prefix
        curPre += str[i];

        # Update Sum
        sum = findSum(curPre, sum);

    # Return Answer
    return sum;

# Driver Code
str = "1225";
print(sumPrefix(str));

# This code is contributed by Saurabh Jaiswal

C#

// C# program of the above approach
using System;

public class GFG{

// Function for finding sum of larger numbers
static String findSum(String str1, String str2)
{

    // Before proceeding further, make
    // sure length of str2 is larger
    if (str1.Length > str2.Length) {
        String s = str1;
        str1=str2;
        str2=s;
    }

    // Stores resulting sum
    String str = "";

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

    // Reverse both of Strings
    str1 = reverse(str1);
    str2 = reverse(str2);

    int carry = 0;
    for (int i = 0; i < n1; i++)
    {

        // Compute sum of current
        // digits and carry
        int sum
            = ((str1[i] - '0')
               + (str2[i] - '0')
               + carry);
        str+=(char)((sum % 10 + '0'));

        // Carry for next step
        carry = sum / 10;
    }

    // Add remaining digits
    for (int i = n1; i < n2; i++) {
        int sum = ((str2[i] - '0') + carry);
        str+=(char)(sum % 10 + '0');
        carry = sum / 10;
    }

    // Add remaining carry
    if (carry > 0)
        str += (carry + '0');

    // Reverse String
    str = reverse(str);

    // Return Answer
    return str;
}

// Function to find sum of all prefixes
// of a String representing a number
static String sumPrefix(String str)
{

    // Stores the desired sum
    String sum = "0";

    // Stores the current prefix
    String curPre = "";

    // Loop to iterate str
    for (int i = 0; i < str.Length; i++)
    {

        // Update current prefix
        curPre += (char)(str[i]);

        // Update Sum
        sum = findSum(curPre, sum);
    }

    // Return Answer
    return sum;
}
static String reverse(String input) {
    char[] a = input.ToCharArray();
    int l, r = a.Length - 1;
    for (l = 0; l < r; l++, r--) {
        char temp = a[l];
        a[l] = a[r];
        a[r] = temp;
    }
    return String.Join("",a);
}

// Driver Code
public static void Main(String[] args)
{
    String str = "1225";
    Console.Write(sumPrefix(str));
}
}

// This code is contributed by 29AjayKumar

Javascript

<script>
        // JavaScript code for the above approach
        // Function for finding sum of larger numbers
        function findSum(str1, str2)
        {

            // Before proceeding further, make
            // sure length of str2 is larger
            if (str1.length > str2.length) {
                let temp = str1;
                str1 = str2;
                str2 = temp;
            }

            // Stores resulting sum
            let str = [];

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

            // Reverse both of strings
            str1 = str1.split('')
            str2 = str2.split('')
            str1.reverse();
            str2.reverse();

            let carry = 0;
            for (let i = 0; i < n1; i++) {
                // Compute sum of current
                // digits and carry
                let sum
                    = ((str1[i].charCodeAt(0) - '0'.charCodeAt(0))
                        + (str2[i].charCodeAt(0) - '0'.charCodeAt(0))
                        + carry);
                str.push(String.fromCharCode(sum % 10 + '0'.charCodeAt(0)));

                // Carry for next step
                carry = Math.floor(sum / 10);
            }

            // Add remaining digits
            for (let i = n1; i < n2; i++) {
                let sum = ((str2[i].charCodeAt(0) - '0'.charCodeAt(0)) + carry);
                str.push(String.fromCharCode(sum % 10 + '0'.charCodeAt(0)));
                carry = Math.floor(sum / 10);
            }

            // Add remaining carry
            if (carry)
                str.push(String.fromCharCode(carry + '0'.charCodeAt(0)));

            // Reverse string
            str.reverse();

            // Return Answer
            return str.join('');
        }

        // Function to find sum of all prefixes
        // of a string representing a number
        function sumPrefix(str)
        {

            // Stores the desired sum
            let sum = "0";

            // Stores the current prefix
            let curPre = "";

            // Loop to iterate str
            for (let i = 0; i < str.length; i++) {
                // Update current prefix
                curPre += str[i];

                // Update Sum
                sum = findSum(curPre, sum);
            }

            // Return Answer
            return sum;
        }

        // Driver Code
        let str = "1225";
        document.write(sumPrefix(str));

       // This code is contributed by Potta Lokesh
    </script>

Output

1360

Time Complexity: O(N²)
Auxiliary Space: O(N)