feat(euler): 20 - Factorial digit sum

-

challenges/euler/readme.md

@@ -31,7 +31,7 @@ The source code can be found in the [src](src) directory. My thoughts about some
 -   [x] [17 - Number letter counts](src/17%20-%20Number%20letter%20counts.ts)
 -   [x] [18 - Maximum path sum I](src/18%20-%20Maximum%20path%20sum%20I.ts)
 -   [x] [19 - Counting Sundays](src/19%20-%20Counting%20Sundays.ts)
--   [ ] 20 - Factorial digit sum
+-   [x] [20 - Factorial digit sum](src/20%20-%20Factorial%20digit%20sum.ts)
 -   [ ] 21 - Amicable numbers
 -   [ ] 22 - Names scores
 -   [ ] 23 - Non-abundant sums

challenges/euler/src/20 - Factorial digit sum.ts

@@ -0,0 +1,31 @@
+// n! means n × (n − 1) × ... × 3 × 2 × 1
+// For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.
+// Find the sum of the digits in the number 100!
+export = {};
+
+/**
+ * Calculate n! as a BigInt
+ */
+const factorial = (n: number) => {
+    if (n < 0) return BigInt(-1);
+    else if (n === 0) return BigInt(1);
+    else return BigInt(BigInt(n) * factorial(n - 1));
+};
+
+/**
+ * Find the sum of the digits in a number
+ */
+const sumOfDigits = (n: number) => {
+    let sum = 0;
+
+    n.toString()
+        .split('')
+        .map(d => parseInt(d))
+        .forEach(digit => (sum += digit));
+
+    return sum;
+};
+
+// Output
+const digitSum = sumOfDigits(factorial(100));
+console.log(digitSum);