Add Factorial in V (#4091)

Author: Kaamkiya

archive/v/v/factorial.v

@@ -0,0 +1,31 @@
+module main
+
+import os
+import strconv
+
+fn factorial(n u64) u64 {
+	return match true {
+		n <= 1 { 1 }
+		else { n * factorial(n-1) }
+	}
+}
+
+fn usage() {
+	println("Usage: please input a non-negative integer")
+	exit(1)
+}
+
+fn main() {
+	if os.args.len != 2 {
+		usage()
+	}
+
+	n := strconv.atoi(os.args[1]) or {
+		usage()
+		exit(1)
+	}
+	if n < 0 {
+		usage()
+	}
+	println(factorial(u64(n)))
+}