add example for stack entry

phrasebook/ch04/generic_data_structure/main.go

@@ -0,0 +1,36 @@
+package main
+
+import "fmt"
+
+type stackEntry struct {
+	next  *stackEntry
+	value interface{}
+}
+
+type stack struct {
+	top *stackEntry
+}
+
+func (s *stack) Push(v interface{}) {
+	var e stackEntry
+	e.value = v
+	e.next = s.top
+	s.top = &e
+}
+
+func (s *stack) Pop() interface{} {
+	if s.top == nil {
+		return nil
+	}
+	v := s.top.value
+	s.top = s.top.next
+	return v
+}
+
+func main() {
+	s := &stack{}
+	s.Push("one")
+	s.Push("two")
+	s.Push("three")
+	fmt.Printf("%#v", s)
+}