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2006-08-04

Another River Crossing Puzzle

Most of us would have seen the standard river crossing puzzle. However, Prasadini recently sent me another river crossing puzzle (Flash animation) that I think is far more challenging.

A father and his two sons, a mother and her two daughters, a thief and a policeman are on one side of a river. There is a boat by the river bank, but it can only take two people at a time. Only the father, the mother and the policeman know how to operate the boat. The father can not be with any of the girls without their mother around. The mother can not be with any of the boys without their father around. The thief can not be with anyone else without the policeman around. How can you get everyone across to the other side of the river?

Start the game by clicking on the big round blue button. Click on a person to get him or her on or off the boat. Click on the levers to get the boat to move.

It took me a fair bit of time to crack it and here is my solution (obfuscated using rot-13):
  1. Gur Guvrs naq gur Cbyvprzna pebff bire.
  2. Gur Cbyvprzna pbzrf onpx (Gur Guvrs qbrf abg eha njnl jura yrsg nybar!).
  3. Gur Cbyvprzna naq n obl pebff bire.
  4. Gur Cbyvprzna naq gur Guvrs pbzr onpx.
  5. Gur Sngure naq gur bgure obl pebff bire.
  6. Gur Sngure pbzrf onpx.
  7. Gur Sngure naq gur Zbgure pebff bire.
  8. Gur Zbgure pbzrf onpx.
  9. Gur Guvrs naq gur Cbyvprzna pebff bire.
  10. Gur Sngure pbzrf onpx.
  11. Gur Sngure naq gur Zbgure pebff bire.
  12. Gur Zbgure pbzrf onpx.
  13. Gur Zbgure naq n qnhtugre pebff bire.
  14. Gur Guvrs naq gur Cbyvprzna pbzr onpx.
  15. Gur Cbyvprzna naq gur bgure qnhtugre pebff bire.
  16. Gur Cbyvprzna pbzrf onpx.
  17. Gur Cbyvprzna naq gur Guvrs pebff bire.
See if you can come up with a better solution.

12 comments:

  1. sorry, could you post the solution in english?

    ReplyDelete
  2. Try it out first before giving up!

    Just copy the solution and paste it into the input area in rot13.com to get the solution in plain English.

    ReplyDelete
  3. There is a simillar riddle on http://ptitclic.net/cassetete/enigme2/index.htm . It's in french but the goal is to cross a river with three priest and three cannibals with a boat. The boat cannot cross empty and cannot contain more than two people. If there are more cannibals than there are priest on one side of the river, then the cannibals eat the priests and you lose. I had to write a (not so) small Prolog program to solve this riddle for uni. There are four possible solutions, all consist of 11 moves if I remember correctly.

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  4. Mathew, I could solve this problem [it took complete concentration of 15 mins]. I also finished the puzzle with 17 steps, the difference your solution and mine is D and S replaced. I don't know whether we could prove solution is not possible in less than 17 moves; it would be nice though.

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  5. Just over a year on, i tried this puzzle as well. After numerous dead-end's, i finally managed to solve it. And yes, i got the same 17 steps; i can't see how it could be done with less.

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  6. Hey, I solved this puzzle in 16 steps. They are:
    01. Policeman and thief cross river.
    02. Policeman returns by self.
    03. Policeman and boy cross river.
    04. Policeman and thief return.
    05. Father and 2nd boy cross river.
    06. Father returns by self.
    07. Father and mother cross river.
    08. Mother returns by self.
    09. Policeman and thief cross river.
    10. Father returns by self.
    11. Father and mother cross river.
    12. Mother returns by self.
    12. Mother and daughter cross river.
    13. Policeman and thief return.
    14. Policeman and girl cross river.
    15. Policeman returns by self.
    16. Policeman and thief cross river.

    ReplyDelete
  7. Not as clever as I thought. It took me 17 moves to solve this puzzle. I double counted step 12. Duh! Ray

    ReplyDelete
  8. That's a simple one,
    Try this version of river ;)

    http://logicgame.com.ua/flash/river.swf

    Have fun
    WIZZARD

    ReplyDelete
  9. is thr a soln fr father daughter puzzle ?

    ReplyDelete
  10. Solved the above 3 girl, 3 father puzzle too :) (http://logicgame.com.ua/flash/river.swf)

    1) G1, F1 cross
    2) F1 stays, G1 comes back
    3) G1, G2 cross
    4) G1 stays, G2 comes back
    5) G2, F2 cross
    6) F2 stays, G2 comes back
    7) G2, G3 cross
    8) G2 stays, G3 comes back
    9) G3, F3 cross

    :)
    Anurag

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  11. this was even more easier......

    ReplyDelete
  12. indeed more easier th father girl game

    ReplyDelete

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