Minimum Recolors to Get K Consecutive Black

lcpy gen -n 2379   # by problem number
lcpy gen -s minimum_recolors_to_get_k_consecutive_black_blocks   # by problem name

Input: blocks = "WBBWWBBWBW", k = 7
Output: 3
Explanation:
One way to achieve 7 consecutive black blocks is to recolor the 0th, 3rd, and 4th blocks
so that blocks = "BBBBBBBWBW".
It can be shown that there is no way to achieve 7 consecutive black blocks in less than 3 operations.
Therefore, we return 3.

Input: blocks = "WBWBBBW", k = 2
Output: 0
Explanation:
No changes need to be made, since 2 consecutive black blocks already exist.
Therefore, we return 0.

class Solution:
    # Time: O(n)
    # Space: O(1)
    def minimum_recolors(self, blocks: str, k: int) -> int:
        whites = sum(1 for c in blocks[:k] if c == "W")
        best = whites
        for i in range(k, len(blocks)):
            if blocks[i] == "W":
                whites += 1
            if blocks[i - k] == "W":
                whites -= 1
            best = min(best, whites)
        return best