question 346 Algorithms & Complexity · medium

Customers who came back

Marketing wants the customers who ordered in January and again in February. Each month's export lists one customer ID per order, so IDs repeat. Write returning_customers(january, february). Exports have tens of thousands of rows; the hidden test counts == comparisons, so don't search a list for every ID.

Functionreturning_customers(january, february)

january, februarylists of customer ID strings, one per order (repeats allowed)

Returnsa sorted list of the IDs that appear in both lists, each once

Constraintat most 2,500 comparisons for two exports of 1,000 IDs

Examples

callreturning_customers(['C00077', 'C01338', 'C00077', 'C00038'], ['C00038', 'C00305', 'C00077'])returns['C00038', 'C00077']

callreturning_customers(['C00001'], ['C00002'])returns[]

callreturning_customers(['C00305', 'C00305'], ['C00305'])returns['C00305']