Compare Value of Current and Previous Row in Spark

I am trying to compare record of current and previous row in the below DataFrame. I want to calculate the Amount column.

scala> val dataset = sc.parallelize(Seq((1, 123, 50), (2, 456, 30), (3, 456, 70), (4, 789, 80))).toDF("SL_NO","ID","AMOUNT")

scala> dataset.show
+-----+---+------+
|SL_NO| ID|AMOUNT|
+-----+---+------+
|    1|123|    50|
|    2|456|    30|
|    3|456|    70|
|    4|789|    80|
+-----+---+------+

Calculation Logic:

1. For the row no 1, AMOUNT should be 50 from first row.
2. For the row no 2, if ID of SL_NO - 2 and 1 is not same then need to consider AMOUNT of SL_NO - 2 (i.e - 30). Otherwise AMOUNT of SL_NO - 1 (i.e. - 50)
3. For the row no 3, if ID of SL_NO - 3 and 2 is not same then need to consider AMOUNT of SL_NO - 3 (i.e - 70). Otherwise AMOUNT of SL_NO - 2 (i.e. - 30)

Same logic need to follow for the other rows also.

Expected Output:

+-----+---+------+
|SL_NO| ID|AMOUNT|
+-----+---+------+
|    1|123|    50|
|    2|456|    30|
|    3|456|    30|
|    4|789|    80|
+-----+---+------+

Please help.