# string <--> int: 
ord('a') = 97, chr(97) = 'a'

'0'.isdecimal() == True

# enumeration:
for i in range(2**n, 2**(n+1)):
  bitmask = bin(i)[3:] # skip '0b1'
  for j in range(n):
    if bitmask[j] == '1':
      # do some actions

# K largest in heap
count = Counter(nums)
return heapq.nlargest(k, count.keys(), key=count.get)

rows = len(grid)  # M
cols = len(grid[0])  # N
dp = [[-1 for _ in range(cols)] for _ in range(rows)]

rows = [0] * n
hills = [0] * (2 * n -1)
dales = [0] * (2 * n -1)

# place
rows[col] = 1
hills[row + col] = 1
dales[row - col] = 1

# remove
rows[col] = 0
hills[row + col] = 0
dales[row - col] = 0

# checking
return not (rows[col] or hills[row + col] or dales[row - col])