Solve the 4 Doors to Hell Riddle

[ashishsinghal]

from each stone numbered n ask " is nth door the door to safety" when he gets consecutive yes or no he will get the right answer. If there are 3 consecutive yes/no then the middle one is the correct gate .ie if he gets answers nynnn then 4th door is the door to safety.

 

[Sybren]

Answer:

Ask each stone if the door corresponding to its number is the safe one.
The stone in front of the safe door can either lie to you, or tell the truth, which doesn't matter because...
The left and right neighbour of the 'safe-door stone' have to do the opposite: if the safe stone lies, the both have to tell the truth, and vice versa. The answers of these three stones will therefore be identical, since the doors neighbouring the safe one are dangerous.

From the list of replies, there will be three consecutive replies identical to each other. The middle one of these three belongs to the safe door.

Example:

safe door: +, dangerous door: -
case I: T,F,T,F,T; case II: F,T,F,T,F

suppose door #3 is safe. The replies will be either:
case I:-+++-
case II:+---+
In both cases the pattern of replies reveals that door #3 is the safe one.

 

[tamtam402]

Edit: Wow nevermind, I'm dumb.