local variable and global variable

package main
var a = "G"
func main() {
   n()
   m()
   n()
}
func n() { print(a) }
func m() {
   a := "O"        // 局部变量
   print(a)
}

GOG

package main
var a = "G"
func main() {
   n()
   m()
   n()
}
func n() { print(a) }
func m() {
   a = "O"        //改变了全局变量
   print(a)
}

GOO