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A helicopter drops two trains, each on a parachute, onto a
straight infinite railway line. There is an undefined
distance between the two trains. Each faces the same
direction, and upon landing, the parachute attached to each
train falls to the ground next to the train and detaches.
Each train has a microchip that controls its motion. The
chips are identical. There is no way for the trains to know
where they are. You need to write the code in the chip to
make the trains bump into each other. Each line of code
takes a single clock cycle to execute.
You can use the following commands (and only these);
MF - moves the train forward
MB - moves the train backward
IF (P) - conditional that's satisfied if the train is next
to a parachute. There is no "then" to this IF statement.
GOTO
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Answer Posted / rshadow
After I was asked this question (two weeks after my comment
#3) in a job interview and was guided to the solution - not
before I tried to give the oscillating solution from comment
#2 above, which can't be coded within the limitation of the
4 available commands - I give here the correct and final answer:
start:
MF
MF
MB
if (p) GOTO found
GOTO start
found:
MF
GOTO found
Explanation:
Both trains start moving in the same direction at the same
rate. After one train meets the parachute of the other
train, it starts going at a faster rate and thus will catch
up with the other train and bump into it.
It's a simple solution, but not that easy to come up with.
Seems to be a popular question in job interviews for
programmers in high-tech companies in Israel (me and a
friend were each asked this riddle at a job interview, in
two different companies).
| Is This Answer Correct ? | 26 Yes | 2 No |
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