update book code

eBook/exercises/chapter_11/stack/stack_general.go

@@ -0,0 +1,39 @@
+// stack.go
+package stack
+
+import "errors"
+
+type Stack []interface{}
+
+func (stack Stack) Len() int {
+	return len(stack)
+}
+
+func (stack Stack) Cap() int {
+	return cap(stack)
+}
+
+func (stack Stack) IsEmpty() bool {
+	return len(stack) == 0
+}
+
+func (stack *Stack) Push(e interface{}) {
+	*stack = append(*stack, e)
+}
+
+func (stack Stack) Top() (interface{}, error) {
+	if len(stack) == 0 {
+		return nil, errors.New("stack is empty")
+	}
+	return stack[len(stack)-1], nil
+}
+
+func (stack *Stack) Pop() (interface{}, error) {
+	stk := *stack	// dereference to a local variable stk
+	if len(stk) == 0 {
+		return nil, errors.New("stack is empty")
+	}
+	top := stk[len(stk)-1]
+	*stack = stk[:len(stk)-1] // shrink the stack
+	return top, nil
+}