第一题就是每次算以下更新的差值，不要去每次都k个求和，代码量也不大

def f(arr, n, k):
    ret_idx = 0
    delta, min_delta = 0, 0
    for i in range(k, n):
        new_delta = delta + arr[i] - arr[i - k]
        if new_delta < min_delta:
            min_delta = new_delta
            ret_idx = i - k + 1
        delta = new_delta
    return ret_idx + 1

哈哈关键是第二题，搞了半天，那个测试用例2为啥是“No”，搞不懂