My solutions for google foobar
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The hard part of this challenge was figuring out how to make it in the time
limit, much like the old XOR challenge. This one though was on another
optimization technique, memoization(I see what you did there Google, challenges
using the skills you ask for interviews on your website).
Hashing the entire list would have been too long, even as a tuple, so instead we
had to be clever and use another trick: building an history of all the moves that
This allows us, given a pair of row and column a and b, to avoid having to do
extreme amount of recursive computation to get a result, as long as an history
of the last (past grid row + current move) moves is saved.
The rest of the problem is fairly easy: we recursively try out all possible
combinations for this grid and save the number of correct ones we could get.
def answer(state, a=0, b=0, past=0, solutions=0, history=0):
past=[[True] * (len(state[0])+1) for i in range(len(state)+1)]
solutions = {}
history = []
return True
index=((a,b), tuple(history[-(len(state)+2):]))
if index in solutions:
return solutions[index]
for cell in [True, False]:
# either all True(c[0][0] and 1 cell) or all False (!c[0][0] and !1 cell)
if (not a or not b) or len(set([((past[a][b-1] + past[a-1][b]
+ past[a-1][b-1] + cell)==1), state[a-1][b-1]]))==1:
past[a][b] = cell
res+=answer(state, a=(a+1)%(len(state)+1),
b=b+(a+1)//(len(state)+1), past=past,
solutions=solutions, history=history)
return res