switch removeFirst to dropFirst

we don't need to alter a mutable copy of `array`, instead we can just drop the first element.

Author: TheIronBorn

Select Minimum Maximum/Minimum.swift

@@ -3,13 +3,12 @@
  */
 
 func minimum<T: Comparable>(_ array: [T]) -> T? {
-    var array = array
     guard !array.isEmpty else {
         return nil
     }
 
-    var minimum = array.removeFirst()
-    for element in array {
+    var minimum = array.first!
+    for element in array.dropFirst() {
         minimum = element < minimum ? element : minimum
     }
     return minimum