学习GO中自定义类型的定义，接口和方法的写法

测试用例： https://github.com/panyingyun/gostudy/blob/master/testStack.go

package main

import (
	"fmt"
	"github.com/panyingyun/gostudy/stacker"
)

func main() {
	a := stacker.NewStack(2)
	a.Push("Google")
	a.Push(100)
	a.Push([]int{1,2,3})
	fmt.Println(a)
	fmt.Println(a.Len())
	fmt.Println(a.Cap())
	for {
		item,err := a.Pop()
		if err != nil {
			fmt.Println(err)
			break
		}
		fmt.Println(item)
	}
}

stack定义： https://github.com/panyingyun/gostudy/blob/master/stacker/stack.go

package stacker

import (
	"errors"
)

type Stack struct {
	st  []interface{}
	len int
	cap int
}

func NewStack(cap int) *Stack {
	st := make([]interface{},cap)
	return &Stack{st,cap}
}

func (s *Stack) Len() int {
	return s.len
}

func (s *Stack) Cap() int {
	return s.cap
}

func (s *Stack) Push(p interface{}) {
	s.st = append(s.st,p)
	s.len = len(s.st)
	s.cap = cap(s.st)
}

func (s *Stack) Pop() (interface{},error) {
	if s.len == 0 {
		return nil,errors.New("Can't pop an empty stack")
	}
	s.len -= 1
	out := s.st[s.len]
	s.st = s.st[:s.len]
	return out,nil
}