Minimum Recolors to Get K Consecutive Black Blocks.py

class Solution:
    """
    Time:   O(n)
    Memory: O(k)
    """

    def minimumRecolors(self, blocks: str, k: int) -> int:
        min_cost = cost = blocks[:k].count('W')

        for i in range(k, len(blocks)):
            cost = cost - (blocks[i - k] == 'W') + (blocks[i] == 'W')
            min_cost = min(min_cost, cost)

        return min_cost