Number of ways to obtain each numbers in range [1, b+c] by adding any two numbers in range [a, b] and [b, c]

Given three integers a, b and c. You need to select one integer from the range [a, b] and one integer from the range [b, c] and add them. The task to calculate the number of ways to obtain the sum for all the numbers in the range [1, b+c].

Examples:

Input: a = 1, b = 2, c = 2
Output: 0, 0, 1, 1
Explanation:
The numbers to be obtained are [1, b+c] = [1, 4] = {1, 2, 3, 4}
Therefore, the number of ways to obtain each are:
1 – can’t be obtained
2 – can’t be obtained
3 – only one way. select {1} from range [a, b] and {2} from range [b, c] – 1 + 2 = 3
4 – only one way. select {2} from range [a, b] and {2} from range [b, c] – 2 + 2 = 4

Input: a = 1, b = 3, c = 4
Output: 0, 0, 0, 1, 2, 2, 1

Simple Approach:



  • A simple brute force solution will be to use a nested loop where exterior loop traverses from i = a to i = b and inner loop from j = b to j = c inclusive.
  • We will initialise array a of size b + c + 1 with zero. Now in loops, we will increment the index at i+j, i.e., (a[i+j]++).
  • We will simply print the array at the end.

Below is the implementation of the above approach.

C++

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// C++ program to calculate
// the number of ways
  
#include <bits/stdc++.h>
using namespace std;
  
void CountWays(int a, int b, int c)
{
    int x = b + c + 1;
    int arr[x] = { 0 };
  
    // Initialising the array with zeros.
    // You can do using memset too.
    for (int i = a; i <= b; i++) {
        for (int j = b; j <= c; j++) {
            arr[i + j]++;
        }
    }
    // Printing the array
    for (int i = 1; i < x; i++) {
        cout << arr[i] << " ";
    }
    cout << endl;
}
// Driver code
int main()
{
    int a = 1;
    int b = 2;
    int c = 2;
  
    CountWays(a, b, c);
  
    return 0;
}

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Java

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// Java program to calculate 
// the number of ways 
class GFG{
      
public static void CountWays(int a, int b,
                                    int c) 
    int x = b + c + 1
    int[] arr = new int[x]; 
      
    // Initialising the array with zeros. 
    // You can do using memset too. 
    for(int i = a; i <= b; i++)
    
       for(int j = b; j <= c; j++)
       
          arr[i + j]++; 
       
    
      
    // Printing the array 
    for(int i = 1; i < x; i++) 
    {
       System.out.print(arr[i] + " ");
    
  
// Driver code
public static void main(String[] args)
{
    int a = 1
    int b = 2
    int c = 2
      
    CountWays(a, b, c); 
}
}
  
// This code is contributed by divyeshrabadiya07

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Python3

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# Python3 program to calculate
# the number of ways
def CountWays(a, b, c):
      
    x = b + c + 1;
    arr = [0] * x;
  
    # Initialising the array with zeros.
    # You can do using memset too.
    for i in range(a, b + 1):
        for j in range(b, c + 1):
            arr[i + j] += 1;
  
    # Printing the array
    for i in range(1, x):
        print(arr[i], end = " ");
          
# Driver code
if __name__ == '__main__':
      
    a = 1;
    b = 2;
    c = 2;
  
    CountWays(a, b, c);
      
# This code is contributed by Rajput-Ji

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C#

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// C# program to calculate 
// the number of ways 
using System;
class GFG{ 
      
public static void CountWays(int a, int b, 
                                    int c) 
    int x = b + c + 1; 
    int[] arr = new int[x]; 
      
    // Initialising the array with zeros. 
    // You can do using memset too. 
    for(int i = a; i <= b; i++) 
    
        for(int j = b; j <= c; j++) 
        
            arr[i + j]++; 
        
    
      
    // Printing the array 
    for(int i = 1; i < x; i++) 
    
        Console.Write(arr[i] + " "); 
    
  
// Driver code 
public static void Main() 
    int a = 1; 
    int b = 2; 
    int c = 2; 
      
    CountWays(a, b, c); 
  
// This code is contributed by rutvik_56

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Output:

0 0 1 1

Time Complexity: O((b-a)*(c-b)), which in the worst case is O(c2)

Efficient Approach: The idea is to use Prefix Sum logic to solve this problem.

  1. We will traverse i from [a, b] and for every i we will simply increment the value of starting interval arr[i + b] by 1 and decrement the value of ending interval arr[i + c + 1] by 1.
  2. Now all we need to do is to calculate the prefix sum of the array ( arr[i]+ = arr[i-1] ) and print the array.

Lets see the approach with the help of an example.
Why does this work?

For example: a = 1, b = 2, c = 2, we will encounter only two values of i
i = 1 = > arr[1+2]++; arr[1+2+1]–;
i = 2 = > arr[2+2]++; arr[2+2+1]–;
arr = {0, 0, 0, 1, 0, -1};
prefix sums:
arr = {0, 0, 0, 1, 1, 0};
Now carefully look and realise that this is our answer.

So what we do at particular index i is arr[i+b]++ and arr[i+c+1]–;

Now we are using prefix sums so all the values will be incremented by 1 between i+b and infinite(We won’t go there but will result in prefix sum increment by 1 and as soon as we do a decrement at i+c+1 all the values between i+c+1 and infinite will be decreased by one.
So effectively all the values in the range [i+b, i+c] are incremented by one, and rest all the values will remain unaffected.

Below is the implementation of the above approach.

C++

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// C++ program to calculate
// the number of ways
  
#include <bits/stdc++.h>
using namespace std;
  
void CountWays(int a, int b, int c)
{
    // 2 is added because sometimes
    // we will decrease the
    // value out of bounds.
    int x = b + c + 2;
  
    // Initialising the array with zeros.
    // You can do using memset too.
    int arr[x] = { 0 };
  
    for (int i = 1; i <= b; i++) {
        arr[i + b]++;
        arr[i + c + 1]--;
    }
  
    // Printing the array
    for (int i = 1; i < x - 1; i++) {
        arr[i] += arr[i - 1];
        cout << arr[i] << " ";
    }
    cout << endl;
}
  
// Driver code
int main()
{
    int a = 1;
    int b = 2;
    int c = 2;
  
    CountWays(a, b, c);
  
    return 0;
}

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Java

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// Java program to calculate
// the number of ways
import java.util.*;
  
class GFG{
  
static void CountWays(int a, int b, int c)
{
      
    // 2 is added because sometimes
    // we will decrease the
    // value out of bounds.
    int x = b + c + 2;
  
    // Initialising the array with zeros.
    int arr[] = new int[x];
  
    for(int i = 1; i <= b; i++)
    {
       arr[i + b]++;
       arr[i + c + 1]--;
    }
  
    // Printing the array
    for(int i = 1; i < x - 1; i++)
    {
       arr[i] += arr[i - 1];
       System.out.print(arr[i] + " ");
    }
    System.out.println();
}
  
// Driver code
public static void main(String[] args)
{
    int a = 1;
    int b = 2;
    int c = 2;
  
    CountWays(a, b, c);
}
}
  
// This code is contributed by Rohit_ranjan

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C#

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// C# program to calculate
// the number of ways
using System;
class GFG{
  
static void CountWays(int a, int b, int c)
{
      
    // 2 is added because sometimes
    // we will decrease the
    // value out of bounds.
    int x = b + c + 2;
  
    // Initialising the array with zeros.
    int []arr = new int[x];
  
    for(int i = 1; i <= b; i++)
    {
        arr[i + b]++;
        arr[i + c + 1]--;
    }
  
    // Printing the array
    for(int i = 1; i < x - 1; i++)
    {
        arr[i] += arr[i - 1];
        Console.Write(arr[i] + " ");
    }
    Console.WriteLine();
}
  
// Driver code
public static void Main()
{
    int a = 1;
    int b = 2;
    int c = 2;
  
    CountWays(a, b, c);
}
}
  
// This code is contributed by Code_Mech

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Output:

0 0 1 1

Time complexity: O(C)

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