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Let's say we have a 2-D list, like the following one:

\$ \begin{bmatrix} \color{red}{1} & \color{red}{2} & \color{red}{3} & \color{red}{4} \\ \color{blue}{5} & \color{blue}{6} & \color{blue}{7} & \color{red}{8} \\ \color{green}{9} & \color{green}{10} & \color{blue}{11} & \color{red}{12} \end{bmatrix} \$

Notice how the top row and the right column is red. For the purposes of this question, let's call this a J-bracket. The 2nd J-bracket is highlighted in blue, and the 3rd J-bracket is highlighted in green. Your challenge is not to find the J-brackets, but given the J-bracket list, you need to return the original 2-D list.

In many cases, there will be more than one 2-D list possible based on the J-brackets. In this case, you can return either of the possible 2-D lists, or you can return a list of the possiblities.

Let's say you are given the list [[1,2,3,4,5,6], [1,2,3,4], [1,2]]. This means the 1st J-bracket is [1,2,3,4,5,6], the 2nd J-bracket is [1,2,3,4], and the 3rd one is [1,2]. There are two different possible matrixes that can be created from these J-brackets:

\$ \begin{bmatrix} \color{red}{1} & \color{red}{2} & \color{red}{3} & \color{red}{4} \\ \color{blue}{1} & \color{blue}{2} & \color{blue}{3} & \color{red}{5} \\ \color{green}{1} & \color{green}{2} & \color{blue}{4} & \color{red}{6} \end{bmatrix} \$

OR:

\$ \begin{bmatrix} \color{red}{1} & \color{red}{2} & \color{red}{3} \\ \color{blue}{1} & \color{blue}{2} & \color{red}{4} \\ \color{green}{1} & \color{blue}{3} & \color{red}{5} \\ \color{green}{2} & \color{blue}{4} & \color{red}{6} \end{bmatrix} \$

Test cases

[[1,2,3,4,5], [1,2,3], [1]] => [[1,2,3], [1,2,4], [1,3,5]]
[[1,2,3,4,5,6], [1,2,3,4], [1,2]] => [[1,2,3], [1,2,4], [1,3,5], [2,4,6]]
                                  OR [[1,2,3,4], [1,2,3,5], [1,2,4,6]]
[[6,8,9,6,5,8,2],[1,7,4,3,2],[2,3,1]] => [[6,8,9,6,5], [1,7,4,3,8], [2,3,1,2,2]]
                                      OR [[6,8,9], [1,7,6], [2,4,5], [3,3,8], [1,2,2]]
[] => []

Rules

✝ The name J-bracket was robbed from this question.

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10 Answers 10

7
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APL(Dyalog Unicode), 20 bytes SBCS

{(↓∘⍺⍪⍵,∘⍪↑∘⍺)-≢⍵}/⊆

Try it on APLgolf!

Nest if simple (if the input is []). This is necessary to deal with the empty case.
/ Reduce the list of J-brackets from right to left with:
{ ... } Takes the current matrix as ⍵ and the new J-bracket as ⍺.
-≢⍵ Number of rows, negated
( ... ) Call the tacit function with this as a right argument.
↑∘⍺ Take that many values from ⍺ (taking a negative amount gets values from the end)
⍵,∘⍪ Append to ⍵ as a column.
↓∘⍺⍪ Drop -≢⍵ values from ⍺, prepend that as a row.  

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6
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J, 33 26 bytes

((}.~-@#),],.({.~-@#))&.>/

Try it online!

A single reduction, where each iteration adds another "J layer". Consider left and right parts like 1 2 3 4 (...) 8 9:

  • ({.~-@#) From the tail of the left list take as many elements as are in the right list:

    3 4
    
  • ],. Zip with right list

    8 3
    9 4
    
  • (}.~-@#), Append the remaining front elements of the left list:

    1 2
    8 3
    9 4
    
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5
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Curry (PAKCS), 60 bytes

f[]=[]
f((a++b):c)=a:f c!b
[]![]=[]
(a:b)!(c:d)=(a++[c]):b!d

Try it online!

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4
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K (ngn/k), 41 40 29 27 26 bytes

{((,i#y),x),'(i:#*x)_y}/|:

Try it online!

Simply aping ovs' APL solution.

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5
  • \$\begingroup\$ Removed a stray byte \$\endgroup\$
    – doug
    Apr 18 at 13:43
  • \$\begingroup\$ @coltim points out that I can simply recurse on the reversed argument. \$\endgroup\$
    – doug
    Apr 18 at 14:01
  • \$\begingroup\$ @coltim Found two more. (He's pretty good at this..) \$\endgroup\$
    – doug
    Apr 18 at 14:06
  • \$\begingroup\$ Stray byte leaked back in. Thanks @PyGamer0. \$\endgroup\$
    – doug
    Apr 18 at 14:11
  • 2
    \$\begingroup\$ The convention here for giving credit for help is, instead of using comments, to update the original post and add something like "-15 bytes thanks to Coltim" right under the TIO link. \$\endgroup\$
    – Jonah
    Apr 18 at 15:16
2
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PARI/GP, 60 bytes

a->matrix(if(w=#a,#a[1]+1-w),w,i,j,a[x=min(w-j+1,i)][i+j-x])

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Returns the one with height ≥ width, e.g. [[1,2,3,4,5,6], [1,2,3,4], [1,2]] => [[1,2,3], [1,2,4], [1,3,5], [2,4,6]].


PARI/GP, 62 bytes

a->matrix(#a,w=if(#a,#a[1]+1-#a),i,j,a[x=min(w-j+1,i)][i+j-x])

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Returns the one with width ≥ height, e.g. [[1,2,3,4,5,6], [1,2,3,4], [1,2]] => [[1,2,3,4], [1,2,3,5], [1,2,4,6]].


In both case, the formula is \$output[i,j]=input[i+\min(w-i-j+1,0),j-\min(w-i-j+1,0)]\$, where \$w\$ is the width of the output matrix.

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2
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Charcoal, 17 bytes

⮌A«Fυ⊞κ⊟ι⊞υι»I⮌υ

Try it online! Link is to verbose version of code. Outputs in Charcoal's default one-element-per-line format. Explanation: Based on my original answer to Find the J twin which has since been superseded by an alternative approach.

⮌A«

Loop over the J-brackets in reverse order.

Fυ⊞κ⊟ι

Append the end elements of the bracket to the existing rows of the predefined empty array.

⊞υι

Append the remainder of the bracket to the predefined empty array.

»I⮌υ

Output the reverse of the final array.

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1
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JavaScript (ES6), 69 bytes

A reduceRight() with a recursive callback function.

a=>a.reduceRight(g=(p,c,i)=>i--?g(p,[...c,a[i].pop()],i):[c,...p],[])

Try it online!

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1
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Python, 69 bytes

f=lambda L:L and L[-1:1]or[*zip(*zip(L[0],*f(L[1:])),L[0][-len(L):])]

Attempt This Online!

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1
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Jelly, 14 bytes

ȯWṚUṭḢ;¥Ɱ¥@/ṚU

Try it online! (The TIO comes with a footer that prints the Python repr, to ensure empty arrays are visible; but the bare link works as a function.)

This is very likely to be suboptimal, since I do four (!) reversals of arrays, but I couldn't find a better solution with Jelly's functions. There could also be a shorter way to handle [] than ȯW, and I might be able to drop the @. However, it feels like this challenge really happened to hit some weak points in Jelly, as the core of the solutions is tiny.

  • The first (and the @) gives us a right-associative reduce. There's no single-byte right-associative reduce in Jelly.
  • The @ also allows us to use / to pop stuff from the new J-bracket, since we can only apply monads to the left arg of dyadic chains.
  • The last reverses the order of the output lines. Since things popped from the J-bracket's end go to the bottom, we have build the output backwards. Also, unfortunately, there's no prepend to like , so we have to append the "rest" of the J-bracket to the bottom of the matrix (; would require a W).
  • The U's reverse each input and output line. ȯWṚṭṪṭ¥Ɱ¥@/Ṛ is very close to correct, but unfortunately gets the innermost J-bracket in the wrong order. I'd need to U or W it somehow to make the output correct.
ȯW                ȯr Wrap (replace [] with [[]])
  Ṛ               Ṛeverse order of J-brackets
   U              Upend (reverse) each J-bracket
           /      reduce lines with:
          @         (swap args: left=next J-bracket, right=matrix)
        Ɱ           Ɱap over lines of matrix:
     Ḣ                remove Ḣead (first) of J-bracket
      ;               append that to matrix line
    ṭ               ṭack (append) remaining J-bracket to matrix
            Ṛ     Ṛeverse order of lines
             U    Upend (reverse) each line
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1
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R, 84 83 bytes

function(j){m[]=unsplit(j,pmin(row(m<-matrix(unlist(j),length(j))),rev(col(m))))
m}

Try it online!

The unsplit strategy corresponding to this answer.

-1 thanks to pajonk.

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2
  • \$\begingroup\$ I was waiting for you to post it :) BTW, -1 byte \$\endgroup\$
    – pajonk
    Apr 19 at 8:54
  • \$\begingroup\$ @pajonk you shouldn't wait. Also this is annoyingly long; perhaps another approach is warranted. \$\endgroup\$
    – Giuseppe
    Apr 19 at 13:43

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