Leetcode 54 (https://leetcode.com/problems/spiral-matrix/)

leetcode_54.ijs

NB. https://leetcode.com/problems/spiral-matrix/

]in=: 1+3 3$i.9
]in2 =: 1+3 4$i.12

walk=: 3 : 0 NB. call with `walk in`
i=. 0 0
d=. 0 NB. R D L U
m=. ($ y) $ 0
o=. a:

tl_bound=. [: +/ 0&>
br_bound=. [: +/ ($y)&<:
bounded=. tl_bound + br_bound

while. (~. , m) -.@-: 1 do.
  o=. o,<i
  m=. 1 (<i)} m
  ni=. i+(d { 0 1,1 0,0 _1,:_1 0) NB. Peek forward
  if. ((m {~ <ni |~ $y) + (bounded ni)) -: 0 do.
    i=. ni
  else.
    d=. 4 | d+1
    ni=. i+(d { 0 1,1 0,0 _1,:_1 0) NB. Peek right
    if. ((m {~ <ni |~ $y) + (bounded ni)) -: 0 do.
      i=. ni
    else.
      break.
    end.
  end.
end.

(}. o) { y
)