Count ways to reach the Nth stair using multiple 1 or 2 steps and a single step 3

Given an integer N number of stairs, the task is count the number ways to reach the N^th stair by taking 1 or 2 step any number of times but taking a step of 3 exactly once.
Examples:

Input: N = 4 
Output: 2 
Explanation:
Since a step of 3 has to be taken compulsorily and only once. So, there are only two possible ways: (1, 3) or (3, 1)
Input: N = 5 
Output: 5 
Explanation:
There are five possible ways to reach 5th stair: (2, 3), (3, 2), (1, 1, 3), (1, 3, 1), (3, 1, 1) 

Approach: This problem can be solved using the concept of Permutations & Combinations. There is a constraint according to which 3 steps at a time have to be taken exactly once. The idea is to count the number of positions that are possible for 3 steps to be taken.
Now, the next task is to find the number of possible two-steps in the movement to N^th stair, which is (N – 3) / 2. In this way, all the possible ways will be covered if the count of two-step is incremented once at a time and for each step, all the possible combinations are calculated.
After Generalising the steps the possible combinations for each possible sequence will be 

Ways = (Length of sequence)! * (Count of 2-step)! 
       ------------------------------------------
                  (Count of 1-step)! 

Algorithm:

• Initialize the length of the sequence to (N – 2).
• Initialize the count of 1-steps to (N – 3).
• Initialize the count of 2-steps to 0
• Initialize the possible count of 2-steps to (N-3)/2.
• Run a loop until the possible count of 2-steps is equal to the actual count of 2-steps. 
  • Find the possible number of ways with the help of the current length of the sequence, count of 1-steps, count of 2-steps and by above-given formulae.
  • Reduce the length of the sequence by 1.
  • Increase the count of 2-steps by 1

Below is the implementation of the above approach.

C++

// C++ implementation to find the number
// the number of ways to reach Nth stair
// by taking 1 or 2 steps at a time and
// 3rd step exactly once
#include <bits/stdc++.h>
using namespace std;

int factorial(int n)
{
    // Single line to find factorial
    return (n == 1 || n == 0) ? 1 :
            n * factorial(n - 1);
}

// Function to find
// the number of ways
int ways(int n)
{
    // Base Case
    if (n < 3)
    {
        return 0;
    }

    // Count of 2-steps
    int c2 = 0;

    // Count of 1-steps
    int c1 = n - 3;

    // Initial length of sequence
    int l = c1 + 1;
    int s = 0;

    // Expected count of 2-steps
    int exp_c2 = c1 / 2;

    // Loop to find the ways for
    // every possible sequence
    while (exp_c2 >= c2)
    {
        int f1 = factorial(l);
        int f2 = factorial(c1);
        int f3 = factorial(c2);
        int f4 = (f2 * f3);

        s += f1 / f4;
        c2 += 1;
        c1 -= 2;
        l -= 1;
    }
    return s;
}

// Driver code
int main()
{
    int n = 7;
    int ans = ways(n);

    cout << ans << endl;
    return 0;
}

// This code is contributed by coder001

Java

// Java implementation to find the number
// the number of ways to reach Nth stair
// by taking 1 or 2 steps at a time and
// 3rd step exactly once
class GFG {

static int factorial(int n)
{

    // Single line to find factorial
    return (n == 1 || n == 0) ? 1 :
            n * factorial(n - 1);
}

// Function to find
// the number of ways
static int ways(int n)
{

    // Base Case
    if (n < 3)
    {
        return 0;
    }

    // Count of 2-steps
    int c2 = 0;

    // Count of 1-steps
    int c1 = n - 3;

    // Initial length of sequence
    int l = c1 + 1;
    int s = 0;

    // Expected count of 2-steps
    int exp_c2 = c1 / 2;

    // Loop to find the ways for
    // every possible sequence
    while (exp_c2 >= c2)
    {
        int f1 = factorial(l);
        int f2 = factorial(c1);
        int f3 = factorial(c2);
        int f4 = (f2 * f3);

        s += f1 / f4;
        c2 += 1;
        c1 -= 2;
         l -= 1;
    }
    return s;
}

// Driver Code
public static void main(String args[])
{
    int n = 7;
    int ans = ways(n);

    System.out.println(ans);
}
}

// This code is contributed by rutvik_56

Python3

# Python3 implementation to find the number
# the number of ways to reach Nth stair
# by taking 1 or 2 steps at a time and
# 3rd Step exactly once

import math

# Function to find
# the number of ways
def ways(n):

    # Base Case
    if n < 3:
        return 0

    # Count of 2-steps
    c2 = 0

    # Count of 1-steps
    c1 = n-3

    # Initial length of sequence
    l = c1 + 1
    s = 0

    # expected count of 2-steps
    exp_c2 = c1 / 2

    # Loop to find the ways for
    # every possible sequence
    while exp_c2 >= c2:
        f1 = math.factorial(l)
        f2 = math.factorial(c1)
        f3 = math.factorial(c2)
        s += f1//(f2 * f3)
        c2 += 1
        c1 -= 2
        l -= 1
    return s

# Driver Code
if __name__ == "__main__":
    N = 7
    print(ways(N))

C#

// C# implementation to find the number
// the number of ways to reach Nth stair
// by taking 1 or 2 steps at a time and
// 3rd step exactly once
using System;
class GFG{

static int factorial(int n)
{

    // Single line to find factorial
    return (n == 1 || n == 0) ? 1 :
            n * factorial(n - 1);
}

// Function to find
// the number of ways
static int ways(int n)
{

    // Base Case
    if (n < 3)
    {
        return 0;
    }

    // Count of 2-steps
    int c2 = 0;

    // Count of 1-steps
    int c1 = n - 3;

    // Initial length of sequence
    int l = c1 + 1;
    int s = 0;

    // Expected count of 2-steps
    int exp_c2 = c1 / 2;

    // Loop to find the ways for
    // every possible sequence
    while (exp_c2 >= c2)
    {
        int f1 = factorial(l);
        int f2 = factorial(c1);
        int f3 = factorial(c2);
        int f4 = (f2 * f3);

        s += f1 / f4;
        c2 += 1;
        c1 -= 2;
        l -= 1;
    }
    return s;
}

// Driver code
static void Main()
{
    int n = 7;
    int ans = ways(n);

    Console.WriteLine(ans);
}
}

// This code is contributed by divyeshrabadiya07

Javascript

<script>
// JavaScript implementation to find the number
// the number of ways to reach Nth stair
// by taking 1 or 2 steps at a time and
// 3rd step exactly once

function factorial(n)
{

    // Single line to find factorial
    return (n == 1 || n == 0) ? 1 :
            n * factorial(n - 1);
}

// Function to find
// the number of ways
function ways(n)
{
    // Base Case
    if (n < 3)
    {
        return 0;
    }

    // Count of 2-steps
    let c2 = 0;

    // Count of 1-steps
    let c1 = n - 3;

    // Initial length of sequence
    let l = c1 + 1;
    let s = 0;

    // Expected count of 2-steps
    let exp_c2 = Math.floor(c1 / 2);

    // Loop to find the ways for
    // every possible sequence
    while (exp_c2 >= c2)
    {
        let f1 = factorial(l);
        let f2 = factorial(c1);
        let f3 = factorial(c2);
        let f4 = (f2 * f3);

        s += Math.floor(f1 / f4);
        c2 += 1;
        c1 -= 2;
        l -= 1;
    }
    return s;
}

// Driver code
    let n = 7;
    let ans = ways(n);
    document.write(ans);

// This code is contributed by Surbhi Tyagi.
</script>

Output:

20

Performance Analysis:

• Time Complexity: As in the above approach, there is one loop to check the possible permutations for each sequence which can go up till (N-3) which takes O(N) time and to find the possible combinations it will take O(N), Hence the Time Complexity will be O(N²).
• Space Complexity: As in the above approach, there is no extra space used, Hence the space complexity will be O(1).

Note: The time complexity can be improved up to O(N) by precomputing the factorials for every number till N.