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Count of different ways to express N as the sum of 1, 3 and 4

  • Difficulty Level : Easy

Given N, count the number of ways to express N as sum of 1, 3 and 4.

Examples: 

Input :  N = 4
Output : 4 
Explanation: 1+1+1+1 
             1+3
             3+1 
             4 

Input : N = 5 
Output : 6
Explanation: 1 + 1 + 1 + 1 + 1
             1 + 4
             4 + 1
             1 + 1 + 3
             1 + 3 + 1
             3 + 1 + 1

Approach : Divide the problem into sub-problems for solving it. Let DP[n] be the be the number of ways to write N as the sum of 1, 3, and 4. Consider one possible solution with n = x1 + x2 + x3 + … xn. If the last number is 1, then sum of the remaining numbers is n-1. So the number that ends with 1 is equal to DP[n-1]. Taking other cases into account where the last number is 3 and 4. The final recurrence would be: 

DPn = DPn-1 + DPn-3 + DPn-4
Base case :
DP[0] = DP[1] = DP[2] = 1 and DP[3] = 2

C++

// C++ program to illustrate the number of
// ways to represent N as sum of 1, 3 and 4.
#include <bits/stdc++.h>
using namespace std;

// function to count the number of
// ways to represent n as sum of 1, 3 and 4
int countWays(int n)
{
    int DP[n + 1];

    // base cases
    DP[0] = DP[1] = DP[2] = 1;
    DP[3] = 2;

    // iterate for all values from 4 to n
    for (int i = 4; i <= n; i++) 
        DP[i] = DP[i - 1] + DP[i - 3] + DP[i - 4];
    
    return DP[n];
}

// driver code
int main()
{
    int n = 10;
    cout << countWays(n);
    return 0;
}

Java

// Java program to illustrate 
// the number of ways to represent 
// N as sum of 1, 3 and 4.

class GFG {

    // Function to count the 
    // number of ways to represent 
    // n as sum of 1, 3 and 4
    static int countWays(int n)
    {
        int DP[] = new int[n + 1];

        // base cases
        DP[0] = DP[1] = DP[2] = 1;
        DP[3] = 2;

        // iterate for all values from 4 to n
        for (int i = 4; i <= n; i++)
            DP[i] = DP[i - 1] + DP[i - 3] 
                    + DP[i - 4];

        return DP[n];
    }

    // driver code
    public static void main(String[] args)
    {
        int n = 10;
        System.out.println(countWays(n));
    }
}

// This code is contributed 
// by prerna saini.

Python3

# Python program to illustrate the number of
# ways to represent N as sum of 1, 3 and 4.

# Function to count the number of
# ways to represent n as sum of 1, 3 and 4
def countWays(n):

    DP = [0 for i in range(0, n + 1)]
    
    # base cases
    DP[0] = DP[1] = DP[2] = 1
    DP[3] = 2

    # Iterate for all values from 4 to n
    for i in range(4, n + 1):
        DP[i] = DP[i - 1] + DP[i - 3] + DP[i - 4]
    
    return DP[n]

    
# Driver code 
n = 10
print (countWays(n))

# This code is contributed by Gitanjali.

C#

// C# program to illustrate 
// the number of ways to represent 
// N as sum of 1, 3 and 4.
using System;

class GFG {

    // Function to count the 
    // number of ways to represent 
    // n as sum of 1, 3 and 4
    static int countWays(int n)
    {
        int []DP = new int[n + 1];

        // base cases
        DP[0] = DP[1] = DP[2] = 1;
        DP[3] = 2;

        // iterate for all values from 4 to n
        for (int i = 4; i <= n; i++)
            DP[i] = DP[i - 1] + DP[i - 3] 
                    + DP[i - 4];

        return DP[n];
    }

    // Driver code
    public static void Main()
    {
        int n = 10;
        Console.WriteLine(countWays(n));
    }
}

// This code is contributed 
// by vt_m.

PHP

<?php
// PHP program to illustrate 
// the number of ways to 
// represent N as sum of 
// 1, 3 and 4.

// function to count the 
// number of ways to 
// represent n as sum of
// 1, 3 and 4
function countWays($n)
{
    $DP = array();

    // base cases
    $DP[0] = $DP[1] = $DP[2] = 1;
    $DP[3] = 2;

    // iterate for all 
    // values from 4 to n
    for ($i = 4; $i <= $n; $i++) 
        $DP[$i] = $DP[$i - 1] + 
                  $DP[$i - 3] + 
                  $DP[$i - 4];
    
    return $DP[$n];
}

// Driver Code
$n = 10;
echo countWays($n);

// This code is contributed
// by Sam007
?>

Javascript

<script>

// Javascript program to illustrate
// the number of ways to represent
// N as sum of 1, 3 and 4.

// Function to count the
// number of ways to represent
// n as sum of 1, 3 and 4
function countWays(n)
{
    var DP = [];
    DP.length = 10;
    DP.fill(0);

    // Base cases
    DP[0] = DP[1] = DP[2] = 1;
    DP[3] = 2;

    // Iterate for all values from 4 to n
    for(var i = 4; i <= n; i++)
        DP[i] = DP[i - 1] + DP[i - 3] + 
                DP[i - 4];

    return DP[n];
}

// Driver code
var n = 10;

document.write(countWays(n));

// This code is contributed by bunnyram19   

</script>

Output: 

64

Time Complexity : O(n) 
Auxiliary Space : O(n)
 

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