You are helping scientists analyze oral history data they have collected by interviewing members of a village.
From these interviews they have learned about a set of n people (all are dead now) whom we will denote P1, P2, ... Pn. They have also collected facts about when these people lived relative to one another. Each fact has one of the following two forms:
For some i and j, person Pi died before person Pj was born, or For some i and j, the life spans of Pi and Pj overlapped at least partially.
Naturally they are not sure that all these facts are correct. What they would like you to determine is whether the data they have collected is at least internally consistent, in the sense that there could have existed a set of people for which all the facts they have learned simultaneously hold.
An efficient algorithm is proposed to do this: either it produces proposed dates of birth and death for each of the n people so that all the facts hold true, or it reports (correctly) that no such dates can exist. That is the facts collected by the scientists are not internally consistent.