add: map exercises and quiz

22-maps/questions/README.md

@@ -0,0 +1,59 @@
+## Why are maps used?
+For example, here is a program that uses a slice to find an element among millions of elements.
+```go
+millions := []int{/* millions of elements */}
+for _, v := range millions {
+    if v == userQuery {
+        // do something
+    }
+}
+```
+1. Maps allow fast-lookup for map keys in O(1) time *CORRECT*
+2. Maps allow fast-lookup for map keys in O(n) time
+3. Maps allow fast-traversal on map keys in O(1) time
+
+> **1:** That's right. Maps work in O(1) in average for fast-lookup.
+>
+> **2:** Map doesn't work in O(n) time for key lookup.
+>
+
+## When shouldn't you use a map?
+1. To find an element through a key
+2. To loop over the map keys *CORRECT*
+3. To add structured data to your program
+
+> **1:** That is when you use a map.
+> 
+> **2:** Right! Looping over map keys happen in O(n) time. So, maps are the worst data structures for key traversing.
+>
+> **3:** Maps don't allow you to add structured data to your program.
+
+
+## Which type below cannot be a map key?
+1. map[string]int
+2. []string
+3. []int
+4. []bool
+5. All of them *CORRECT*
+
+> **5:** Slices, maps, and function values are not comparable. So, they cannot be map keys.
+>
+
+## Which are the following map's key and element types?
+```go
+map[string]map[int]bool
+```
+1. Key: string Element: bool
+2. Key: string Element: int
+3. Key: string Element: map[int]
+4. Key: string Element: map[int]string *CORRECT*
+
+> **4:** The map contains other maps. The element type of a map can be of any type.
+>
+
+## What is a map behind the scenes?
+1. A map header
+2. A pointer to a map header *CORRECT*
+3. Tiny data structure with 3 fields: Pointer, Length and Capacity
+
+> **2**: That's right. Maps are complex data structures. However, each map value is only a pointer to a map header (which is a more complex data structure).