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
jenishmonpara: 2020-04-20 15:35:48

My first AC in one go in SPOJ.
Some 2D DP and BFS

Last edit: 2020-04-20 15:36:24
nagaraj41: 2020-04-13 08:38:54

just google 2^31 -1 and copy paste if you are having issues. Also take care of the output format. i typed "THIS" instead of "THE" resulting in WA's. felt foolish.

coolio_1: 2020-02-19 17:16:39

Multiple WA due to MOD.
ll MOD = ((ll)1<<31) - 1;

pas7582128: 2019-12-30 19:06:44

AC in one go.

sudhiawasthi: 2019-11-14 11:10:24

For people getting WA on test case 18, check how your bfs works for the first row and first column, the invalid moves are causing WA.

wingman__7: 2019-11-09 18:09:41

easy BFS problem, use queue no need for recursion

ekta_9: 2019-11-09 07:24:47

i am using top -down dp here but still getting TLE .whyyyyy?

darshanpatel: 2019-05-22 07:29:26

You have to actually print "THE GAME IS A LIE " or " INCONCEIVABLE" (without quotes) ! Take care of modulo by typecasting 1<<31 to ((long long)1<<31).

sky_scraper: 2019-05-12 14:29:29

Damn the modulo just gave me a headache use (1ll<<31)-1 instead of ll(1<<31) - 1 and here "ll" == long long

vengatesh15: 2017-03-25 09:23:46

dfs tle .. bfs AC..

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