Challenge 1: An API Interface — Solution

data class Post(
    val id: Int,
    val title: String,
    val body: String
)

interface PostApiService {
    @GET("posts")
    suspend fun getPosts(): List<Post>

    @GET("posts/{id}")
    suspend fun getPost(@Path("id") id: Int): Post
}

val retrofit = Retrofit.Builder()
    .baseUrl("https://jsonplaceholder.typicode.com/")
    .addConverterFactory(MoshiConverterFactory.create())
    .build()

val postApiService: PostApiService = retrofit.create(PostApiService::class.java)

Notes:
- getPosts() has no @Path parameter — it maps to a plain GET
  https://jsonplaceholder.typicode.com/posts request.
- getPost(id: Int) uses @Path("id") to substitute the id parameter into
  the "{id}" placeholder in "posts/{id}" — calling getPost(5) requests
  https://jsonplaceholder.typicode.com/posts/5.
- Post's property names (id, title, body) happen to match
  jsonplaceholder's actual JSON field names exactly, so no @Json/
  @SerializedName renaming annotation is needed here — that's only
  required when a JSON key and the desired Kotlin property name differ.
- retrofit.create(PostApiService::class.java) generates a real working
  implementation of the interface at runtime — no method body was ever
  written for getPosts() or getPost().