How should I practice Count Repeat Customers Across Dates?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Amazon phone screen problems in FastPrep.

Is Count Repeat Customers Across Dates associated with Amazon?

FastPrep tags this as a Amazon phone screen coding practice problem when the problem catalog has that company and stage context.

Count Repeat Customers Across Dates

Amazon● MediumFULLTIMEPHONE SCREEN

Learn

Problem statement

You are given an array visits. Each row contains exactly two strings: [customerId, timestamp]. Every timestamp is a valid UTC value in the fixed format YYYY-MM-DDTHH:MM.

Only visits whose timestamps are between windowStart and windowEnd, inclusive, belong to the reporting window.

A customer is a repeat customer when that customer has at least one visit on each of two different calendar dates inside the reporting window. Multiple visits by the same customer on the same date count once for this rule.

Return the number of repeat customers.

Function

countRepeatCustomers(visits: String[][], windowStart: String, windowEnd: String) → int

Examples

Example 1

visits = [["alice","2026-08-18T09:00"],["bob","2026-08-18T09:15"],["alice","2026-08-19T10:00"],["bob","2026-08-18T17:00"],["cara","2026-08-20T12:00"]]windowStart = "2026-08-18T00:00"windowEnd = "2026-08-19T23:59"return = 1

alice visits on August 18 and August 19 inside the window. Both of bob's visits are on August 18, and cara's visit is outside the window.

Example 2

visits = [["a","2026-08-01T00:00"],["a","2026-08-02T23:59"],["b","2026-08-02T23:59"],["b","2026-08-03T00:00"],["c","2026-08-02T12:00"]]windowStart = "2026-08-01T00:00"windowEnd = "2026-08-02T23:59"return = 1

The window includes both boundary timestamps. Customer a appears on two dates. Customer b's August 3 visit is outside the window, and c appears on one date.

Example 3

visits = [["u1","2026-09-01T08:00"],["u1","2026-09-01T09:00"],["u2","2026-09-01T10:00"],["u2","2026-09-02T10:00"]]windowStart = "2026-09-01T00:00"windowEnd = "2026-09-02T23:59"return = 1

Two visits by u1 on the same date do not make that customer repeat. Customer u2 visits on two different dates.

Constraints

0 <= visits.length <= 200000.
Every row in visits contains exactly two non-empty strings: a customer ID and a timestamp.
Every customer ID contains at most 64 English letters, digits, hyphens, or underscores.
Every timestamp, windowStart, and windowEnd is a valid UTC value in the fixed format YYYY-MM-DDTHH:MM.
windowStart <= windowEnd.
The visits may be in any order.