Solution to Problem 20

Cargo.toml

@@ -4,3 +4,5 @@ version = "0.1.0"
 authors = ["ghost <mattgathu@gmail.com>"]
 
 [dependencies]
+num-bigint = "0.2.0"
+num-traits = "0.2.5"

src/bin/20.rs

@@ -0,0 +1,38 @@
+// 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!
+
+extern crate num_bigint;
+extern crate num_traits;
+
+use num_bigint::BigUint;
+
+
+/// Computes the Factorial the BigUint type
+fn factorial(num: u32) -> BigUint {
+    (1..=num).map(BigUint::from).product()
+}
+
+fn problem_20_sol_1(limit: u32) -> u32 {
+    let sum_str = format!("{}", factorial(limit));
+    sum_str.chars().map(|c| c.to_digit(10).unwrap()).sum()
+}
+
+fn main() {
+    let limit = 100;
+    println!("Sum of the digits in the number {}!: {}",
+             limit, problem_20_sol_1(100));
+}
+
+#[test]
+fn test_factorial() {
+    assert_eq!(BigUint::from_u32(3628800).unwrap(), factorial(10));
+}
+
+#[test]
+fn test_problem_20_sol_1() {
+    assert_eq!(27, problem_20_sol_1(10));
+}