minimum-recolors-to-get-k-consecutive-black-blocks.py

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

# sliding window
class Solution(object):
    def minimumRecolors(self, blocks, k):
        """
        :type blocks: str
        :type k: int
        :rtype: int
        """
        result = k
        curr = 0
        for i, x in enumerate(blocks):
            curr += int(blocks[i] == 'W')
            if i+1-k < 0:
                continue
            result = min(result, curr)
            curr -= int(blocks[i+1-k] == 'W')
        return result