solved some tasks from Collections part

Author: amullabaev

Collections/Get used to new style/src/Task.kt

@@ -1,8 +1,8 @@
 fun doSomethingStrangeWithCollection(collection: Collection<String>): Collection<String>? {
 
-    val groupsByLength = collection. groupBy { s -> TODO() }
+    val groupsByLength = collection.groupBy { s -> s.length }
 
-    val maximumSizeOfGroup = groupsByLength.values.map { group -> TODO() }.max()
+    val maximumSizeOfGroup = groupsByLength.values.map { group -> group.size }.max()
 
-    return groupsByLength.values.firstOrNull { group -> TODO() }
+    return groupsByLength.values.firstOrNull { group -> group.size == maximumSizeOfGroup }
 }

Collections/GroupBy/src/Task.kt

@@ -1,2 +1,2 @@
 // Return a map of the customers living in each city
-fun Shop.groupCustomersByCity(): Map<City, List<Customer>> = TODO()
+fun Shop.groupCustomersByCity(): Map<City, List<Customer>> = customers.groupBy { it.city }

Collections/Max min/src/Task.kt

@@ -1,5 +1,8 @@
 // Return a customer whose order count is the highest among all customers
-fun Shop.getCustomerWithMaximumNumberOfOrders(): Customer? = TODO()
+fun Shop.getCustomerWithMaximumNumberOfOrders(): Customer? = customers.maxBy { it.orders.size }
 
 // Return the most expensive product which has been ordered
-fun Customer.getMostExpensiveOrderedProduct(): Product? = TODO()
+fun Customer.getMostExpensiveOrderedProduct(): Product? {
+    val products = orders.flatMap { it.products }
+    return products.maxBy { it.price }
+}

Collections/Sort/src/Task.kt

@@ -1,2 +1,2 @@
 // Return a list of customers, sorted by the ascending number of orders they made
-fun Shop.getCustomersSortedByNumberOfOrders(): List<Customer> = TODO()
+fun Shop.getCustomersSortedByNumberOfOrders(): List<Customer> = customers.sortedBy { it.orders.size }

Collections/Sum/src/Task.kt

@@ -1,3 +1,3 @@
 // Return the sum of prices of all products that a customer has ordered.
 // Note: the customer may order the same product for several times.
-fun Customer.getTotalOrderPrice(): Double = TODO()
+fun Customer.getTotalOrderPrice(): Double = orders.flatMap { it.products }.sumByDouble { it.price }