Added Stack.Peek.

stack.go

@@ -43,3 +43,10 @@ func (s Stack[T]) Empty() bool {
 func (s *Stack[T]) PushMany(ts ...T) {
 	s.items = append(s.items, ts...)
 }
+
+// Peek returns the same value as Pop, but does not remove it from the Stack.
+//
+// Peek panics when called on an empty Stack.
+func (s *Stack[T]) Peek() T {
+	return s.items[len(s.items)-1]
+}

stack_test.go

@@ -26,18 +26,21 @@ func TestThrees(t *testing.T) {
 		gotest.Expect(t, n/3+2, s.Len())
 		gotest.Require(t, !s.Empty())
 
+		gotest.Expect(t, n+1, s.Peek())
 		gotest.Expect(t, n+1, s.Pop())
 		gotest.Expect(t, n/3+1, s.Len())
 		gotest.Require(t, !s.Empty())
 	}
 
 	for n := 27; n >= 0; n -= 3 {
+		gotest.Expect(t, n, s.Peek())
 		gotest.Expect(t, n, s.Pop())
 	}
 
 	gotest.Expect(t, 0, s.Len())
 	gotest.Require(t, s.Empty())
 	gotest.MustPanic(t, func() { s.Pop() })
+	gotest.MustPanic(t, func() { s.Peek() })
 }
 
 func TestPushMany(t *testing.T) {