Find the largest N digit number in a W by H grid of digits

Question

Write a program or function that takes in a positive integer N and a grid of decimal digits (0 to 9) with width W and height H (which are also positive integers). You can assume that N will be less than or equal to the larger of W and H.

Print or return the largest contiguous N digit number that appears horizontally or vertically in the grid, written in normal reading order or in reverse.

• Diagonal lines of digits are not considered.
• The grid does not wrap around, i.e. it does not have periodic boundary conditions.

For example, the 3×3 grid

928
313
049

would have 9 as the output for N = 1, 94 as the output for N = 2, and 940 as the output for N = 3.

The 4×3 grid

7423
1531
6810

would have 8 as the output for N = 1, 86 for N = 2, 854 for N = 3, and 7423 for N = 4.

The 3×3 grid

000
010
000

would have output 1 for N = 1, and 10 for N = 2 and N = 3 (010 is also valid for N = 3).

The 1×1 grid

0

would have output 0 for N = 1.

You can take the input in any convenient reasonable format. e.g. the grid could be a newline separated string of digits, or a multidimensional array, or a list of lists of digits, etc. Leading zeros are allowed in the output if they were part of the grid.

This is code-golf, so the shortest code in bytes wins, but I'll also award brownie points (i.e. more likely upvotes) for answers that can show that their algorithm is computationally efficient.

Answer 1

Pyth, 22 19 bytes

3 bytes thanks to Jakube.

seSs.:RQ.n,L_MdCB.z

Try it online.

If we are allowed to print leading zeroes, the code is 18 bytes:

eSs.:RQ.n,L_MdCB.z

Answer 2

CJam, 39 36 35 34 bytes

qN/)i\[{zW%_}4*]ff{_,@e<ew:i}e_:e>

Just quickly, before @Dennis wakes up :P

Try it online.

Explanation

The basic algorithm is to take all four rotations of the grid and split each row into chunks of length N (or the row length, whichever's smaller). Then convert the chunks to ints and take the largest.

qN/             Split input by newlines, giving an array of lines
)i\             Drop N from the array and put at bottom
[        ]      Wrap in array...
 {    }4*         Perform 4 times...
  zW%_              Rotate grid anticlockwise and push a copy
                Note that this gives an array of 5 grids [CCW1 CCW2 CCW3 CCW4 CCW4]
ff{         }   For each grid row, mapping with N as an extra parameter...
   _,             Push length of row
     @e<          Take min with N
        ew        Split into chunks
          :i      Convert to ints
e_              Flatten that array
:e>             Take cumulative max

Answer 3

Burlesque

Not a final answer yet but it probably will work like this:

blsq ) "7423\n1531\n6810"ln)XXJ)\[jtp)\[_+J)<-_+{3.+ti}m[>]
854
blsq ) "7423\n1531\n6810"ln)XXJ)\[jtp)\[_+J)<-_+{4.+ti}m[>]
7423

How is N and the grid given exactly?