Can we save them? Beta Rabbit is trying to break into a lab that contains the only known zombie cure – but there’s an obstacle. The door will only open if a challenge is solved correctly. The future of the zombified rabbit population is at stake, so Beta reads the challenge: There is a scale with an object on the left-hand side, whose mass is given in some number of units. Predictably, the task is to balance the two sides. But there is a catch: You only have this peculiar weight set, having masses 1, 3, 9, 27, … units. That is, one for each power of 3. Being a brilliant mathematician, Beta Rabbit quickly discovers that any number of units of mass can be balanced exactly using this set.
To help Beta get into the room, write a method called answer(x), which outputs a list of strings representing where the weights should be placed, in order for the two sides to be balanced, assuming that weight on the left has mass x units.
The first element of the output list should correspond to the 1-unit weight, the second element to the 3-unit weight, and so on. Each string is one of:
“L” : put weight on left-hand side
“R” : put weight on right-hand side
“-” : do not use weight
To ensure that the output is the smallest possible, the last element of the list must not be “-“.
x will always be a positive integer, no larger than 1000000000.
(int) x = 2
(string list) [“L”, “R”]
(int) x = 8
(string list) [“L”, “-“, “R”]
from itertools import combinations def answer(x): """ Main logic: find the closest number y >= x so such that: 1. y is a sum of powers of 3 2. x + z = y where z is sum of numbers, each of which is a power of 3 3. z will be decomposed to "L" and y will be decomposed to "R" in build_list() """ next = find_next_sum_of_powers(x) left = represent_as_powers(next - x) while list(set(left)) != left: # meaning we have the same item more than once in "left" # look for the next "sum of powers" and try to use it instead next = find_next_sum_of_powers(next + 1) left = represent_as_powers(next - x) right = represent_as_powers(next) return build_list(left, right) def find_next_sum_of_powers(x): """ Given a number x, find the closest number y such that y >= x and y is one of the numbers returned by get_list_of_sums() and there is no number z, which is also returned by get_list_of_sums() such that: y > z > x """ next = find_next_power(x) if x == next: return x if x <= next // 2: sums =  for list_of_sums in get_list_of_sums(x): for sm in list_of_sums: if sm not in sums: if x == sm: return x if sm - x in sums: return sm sums.append(sm) else: return next def is_power_of_3(x): """ Checks if x is a power of 3 """ n = 0 b = 0 while n < x: n = 3 ** b b += 1 return n == x def get_list_of_sums(x): """ Generates a list of sums of powers of 3: 1, 3, 4, 9, 10, 12, 13... 4 = 3 + 1; 10 = 9 + 1; 12 = 9 + 3; 13 = 9 + 3 + 1; ... """ workset =  for i in gen_next_power(): if i > x: break workset.append(i) res =  for i in range(len(workset)+1): for j in combinations(workset, i): res.append(j) yield map(sum, res[1:]) def gen_next_power(): """ Generator for powers of 3 each iteration will get the next power """ pow = 0 while True: yield 3 ** pow pow += 1 def find_next_power(x): """ Given a number x find the closest power of 3, y such that y > x and there is no power of 3, z such that x < z < y """ if x == 1: return x t = 3 while t < x: t *= 3 return t def represent_as_powers(num): res =  while num > 0: t = find_next_power(num) if t > num: t /= 3 res.append(t) num -= t return res[::-1] def build_list(left, right): stop = right[-1] res, i = , 0 while 3 ** i <= stop : if 3 ** i in left: res.append("L") elif 3 ** i in right: res.append("R") else: res.append("-") i += 1 return res
def answer(x): return to_balanced(to_ternary(x)) def to_ternary(x): """ convert decimal into ternary """ res = "" while x > 0: res = str(x % 3) + res x /= 3 return int(res) def to_balanced(t): """ convert ternary number into balanced-ternary and then to L/-/R representation """ res = map(int, str(t))[::-1] # iterating the digit from right --> left for i in range(0, len(res)): # zeros will remain zeros and ones remain ones # the only thing we need to convert is 2 into +- (== +1-1) if res[i] == 2: res[i] = -1 res = inc(res, i + 1) # convert from (1,0,-1) representation to (R,-,L) return ['-' if x == 0 else ('R' if x == 1 else 'L') for x in res] def inc(arr, ind): """ calculate the carryover """ if len(arr) == ind: arr +=  # prepend the last carryover elif arr[ind] == -1: arr[ind] = 0 elif arr[ind] == 0: arr[ind] = 1 elif arr[ind] == 1: arr[ind] = -1 arr = inc(arr, ind + 1) else: # arr[ind] == 2 arr[ind] = 0 arr = inc(arr, ind + 1) return arr
2 thoughts on “foo.bar: Peculiar balance”
I recently got this challenge. Initially, I figured this was relatively easy, as it is pretty simple to do in your head (for small numbers anyway). The path I took was to break down the what I was doing in my head in a fraction of a second into individual steps in my head. Easier said than done. What I ended up with was to go with the power of 3 that was closest to the difference between the total weight of the right and left sides. Unfortunately, this only ended up working about 70% of the time. I could not come up with a hard and fast rule to go higher or lower than my target.
So in the end I used simple probability and brute force to solve it. I simply use a random number, 0 or 1, to decide on higher or lower, looping until I get it right. Given the maximum x value of 1,000,000,000, that’s somewhere around 1 million combinations. In my testing it only took a few thousand tries, though. But I get the correct answer, every time.
Upon verifying my answer in Google’s environment, however, I realized that Python’s random library is not supported, even though they don’t say so. I even tried os.urandom, but that wasn’t supported either.
I was trying really hard to not search for information on this specific problem as I wanted to do it on my own. Unfortunately I no longer have time left to fully understand this ternary concept and roll my own. Oh well, maybe next time.
Thank you for posting this. It’s eye opening. Will certainly be playing with the concept in the future.
Thanks a lot for this! I’m far from perfect at maths and was not having a good time with this question. Your explanation really helped a lot!