ROBOTGRI - Robots on a grid

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You have recently made a grid traversing robot that can
nd its way from the top left corner of a grid to the
bottom right corner. However, you had forgotten all your
AI programming skills, so you only programmed your robot
to go rightwards and downwards (that's after all where the
goal is). You have placed your robot on a grid with some
obstacles, and you sit and observe. However, after a while
you get tired of observing it getting stuck, and ask yourself
\How many paths are there from the start position to the
goal position?", and \If there are none, could the robot have
made it to the goal if it could walk upwards and leftwards?"
So you decide to write a program that, given a grid of
size n x n with some obstacles marked on it where the robot
cannot walk, counts the di erent ways the robot could go
from the top left corner s to the bottom right t, and if none,
tests if it were possible if it could walk up and left as well.
However, your program does not handle very large numbers, so the answer should be
given modulo 2^31 - 1.


On the rst line is one integer, 1 < n <= 1000. Then follows n lines, each with n characters,
where each character is one of '.' and '#', where '.' is to be interpreted as a walkable
tile and '#' as a non-walkable tile. There will never be a wall at s, and there will never
be a wall at t.


Output one line with the number of di erent paths starting in s and ending in t (modulo
2^31 - 1) or THE GAME IS A LIE if you cannot go from s to t going only rightwards and
downwards but you can if you are allowed to go left and up as well, or INCONCEIVABLE if
there simply is no path from s to t.


..... Output: 6

hide comments
Mitch Schwartz: 2014-07-09 00:26:25

@Andres Felipe Ruiz Cardozo: Judging does not halt on first failure, the way SPOJ is currently set up. (You can see for yourself by submitting infinite loop code.)

Andres Felipe Ruiz Cardozo: 2014-07-08 22:49:25

This problem is frustrating!! TLE on test 18 and don't know what else to tinker with.

EDIT: Actually I am an idiot. It's entirely my fault!

Last edit: 2014-07-09 00:35:16
Black Hole: 2014-04-20 05:19:30

finally AC. same code but one is TLE, the other is AC :))

newbie: 2014-02-11 17:43:03

finally AC

Titas Skrebė: 2014-01-28 17:50:09

be very careful with that goddamn modulo.

Ankur Ahuja: 2013-09-22 21:39:16

What's the case with the 18th test case ? WHY TLE ? Any suggestions ? Hints ?

BLANKRK: 2013-07-11 18:04:45

huhh.......finaly many runtime errrorss.....

Mayank Thakur: 2013-05-29 08:33:18

WA in 18th case even after taking care of (2^31-1)%(2^31-1) :/
when corrected the mistake its giving runtime on 15th...sadness

Last edit: 2013-05-29 13:02:27
vishal goel: 2013-03-30 14:57:31

check modulus

Last edit: 2014-01-17 11:42:26
Janani: 2013-03-26 18:18:12

Classic problem! Take care of overflows, and keep in mind what you will get when you do (1<<31-1)%(1<<31-1).

Last edit: 2013-03-26 18:18:33

Added by:Krzysztof Lewko
Time limit:1s
Source limit:50000B
Memory limit:1536MB
Cluster: Cube (Intel G860)
Languages:All except: ASM64
Resource:Nordic programming contest