Day 9: Recursion 3

Objective

Today, we’re learning and practicing an algorithmic concept called Recursion. Check out the Tutorial tab for learning materials and an instructional video!

Recursive Method for Calculating Factorial

Task

Write a factorial function that takes a positive integer, N as a parameter and prints the result of N! (N factorial).

Note: If you fail to use recursion or fail to name your recursive function factorial or Factorial, you will get a score of 0.

Input Format

A single integer, N (the argument to pass to factorial).

Constraints

• 2 <= N <= 12
• Your submission must contain a recursive function named factorial.

Output Format

Print a single integer denoting N!.

Sample Input

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3

Sample Output

1

6

Explanation

Consider the following steps:

1. factorial(3) = 3 X factorial(2)
2. factorial(2) = 2 X factorial(1)
3. factorial(1) = 1

From steps 2 and 3, we can say factorial(2) = 2 X 1 = 2; then when we apply the value from factorial(2) to step 1, we get factorial(3) = 3 X 2 X 1 = 6. Thus, we print 6 as our answer.

Solutions

Solution 1

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// Complete the factorial function below.
function factorial(n) {
    return (!+n) ? 1 : n * factorial(n - 1);
}

Solution 2

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// Complete the factorial function below.
let memoization = [0, 1];

const factorial = (n) => {
    (typeof memoization[n] !== 'number') && (
        memoization[n] = (n - 1) > 0 ? n * factorial(n - 1) : 1 
    );

    return memoization[n];
}