Maximum Concurrent Calls

Problem A telecom provider bills customers according to their call system usage. The billing department needs to know, for each customer, the maximum number of simultaneous calls that customer had on each day. You receive a list of Call records. Every record describes a single phone call and holds these fields: customerId: an integer that identifies the customer. callId: a unique string that identifies the call. startTimestamp: the instant the call began, given as a Unix…