finish 20

src/euler20/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "euler20"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+rust-gmp = "*"

src/euler20/src/main.rs

@@ -0,0 +1,19 @@
+use gmp::mpz::Mpz;
+
+fn main() {
+    let x = faculty(Mpz::from(100));
+    let y: u32 = x
+        .to_string()
+        .chars()
+        .map(|c| c.to_digit(10).unwrap() as u32)
+        .sum();
+    println!("{y}");
+}
+
+fn faculty(x: Mpz) -> Mpz {
+    if x == Mpz::one() {
+        x
+    } else {
+        x.clone() * faculty(x - 1)
+    }
+}