Minimum Number of Swaps to Make the Binary String Alternating — Python

# Time:  O(n)
# Space: O(1)

class Solution(object):
    def minSwaps(self, s):
        """
        :type s: str
        :rtype: int
        """
        def cost(s, x): 
            diff = 0 
            for c in s:
                diff += int(c) != x
                x ^= 1
            return diff//2
    
        ones = s.count('1')
        zeros = len(s)-ones 
        if abs(ones-zeros) > 1:
            return -1
        if ones > zeros:
            return cost(s, 1)
        if ones < zeros:
            return cost(s, 0)
        return min(cost(s, 1), cost(s, 0))