Cf:d. black and white stripe

analysis

Every time l and r Forward one
Analyze the elements of the head and tail , To get the minimum number in the current window

Ac code

for t in range(int(input())):
    #print(str(t) + ':')
    n, k = list(map(int, input().split()))
    s = input()

    cntW = 0
    ans = 0xffffffff
    for i in range(k):
        if s[i] == 'W':
            cntW += 1
    ans = min(ans, cntW)

    for i in range(k, n):
        if s[i] == 'W' and s[i - k] == 'W':
            pass
        elif s[i] == 'W' and s[i - k] == 'B':
            cntW += 1
        elif s[i] == 'B' and s[i - k] == 'W':
            cntW -= 1
        elif s[i] == 'B' and s[i - k] == 'B':
            pass
        ans = min(ans, cntW)
    print(ans)

summary

Thinking Shifts + The sliding window