EC 9. In the desert there is a jail. After rebellion the single guard has been immobilized. He watched helplessly how prisoners left one by one with different speed in different directions. Later he free himself and took a motobike with extra seat. Now he can follow footprints and pick up one prisoner at a time and bring them back to jail. Each prisoner moves with constant individual speed v_i and left the jail at time t_i. In which order the guard should bring the prisoners back in order to minimize the time?

Database System Concepts
7th Edition
ISBN:9780078022159
Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan
Chapter1: Introduction
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EC 9. In the desert there is a jail. After rebellion
the single guard has been immobilized. He
watched helplessly how prisoners left one by one
with different speed in different directions. Later
he free himself and took a motobike with extra
seat. Now he can follow footprints and pick up
one prisoner at a time and bring them back to jail.
Each prisoner moves with constant individual
speed v_i and left the jail at time t_i. In which
order the guard should bring the prisoners back
in order to minimize the time?
Transcribed Image Text:EC 9. In the desert there is a jail. After rebellion the single guard has been immobilized. He watched helplessly how prisoners left one by one with different speed in different directions. Later he free himself and took a motobike with extra seat. Now he can follow footprints and pick up one prisoner at a time and bring them back to jail. Each prisoner moves with constant individual speed v_i and left the jail at time t_i. In which order the guard should bring the prisoners back in order to minimize the time?
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Solution:

 

We will solve this with greedy algorithm. At any given time instant 't' measure the value d_i_t = v_i * (t - t_i) for each of the prisoners. Choose the maximum value amongst these and go and catch that prisoner. For prisoners having the same value for d_i_t , go for the prisoner who has higher speed, else he will move more distance and the guard will take more time in catching that prisoner.

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