In scala how to combine List of Map, of unequal size based on common keys with same value

Question

Input:

val list1 = List(
    Map("ID" -> "123", "NAME" -> "P1", "PID" -> "321"),
    Map("ID" -> "456", "NAME" -> "P2",  "PID" -> "333")
)

val list2 = List(
    Map("ID" -> "123", "ADDRESS" -> "A1")
)

val list3 = List(
    Map("PID" -> "321", "PHONE" -> "2222"), 
    Map("PID" -> "789", "PHONE" -> "4444")
)

Output:

List(
    Map(
        "ID" -> "123", 
        "NAME" -> "P1", 
        "PID" -> "321", 
        "ADDRESS" -> "A1", 
        "PHONE" -> "2222"
    )
)

I tried iterating list over flatmate and map, but it had bad time complexity. Expecting a solution in more functional programming approach, not using loop. Thanks in advance for helping with providing solution.

list1
  .flatMap { l1 =>
    list2
      .flatMap { l2 =>
        list3
          .map { l3 =>

            if ((l1.ID === l2.ID) && (l1.PID === l3.PID)) {

              val data = Map(
                "ID" -> l1.ID,
                "NAME" -> l1.NAME,
                "PID" -> l1.PID,
                "ADDRESS" -> l2.ADDRESS,
                "PHONE" -> l3.PHONE,
              )

              val temp = List(data)
              temp              
            }

          }
      }
  }

Answer 1

(list1 ++ list2 ++ list3).reverse.reduce(_ ++ _)

This creates a single list of Maps and then reduces them to a single Map. The reverse is needed so that the earlier values take precedence.