i = j = 0
while i < N:
    while j < A[i] + 1:
        unite(i - j, i + j)
        j += 1

    k = 1
    while i - k >= 0 and k + A[i - k] + 1 < j:
        k += 1
    i += k
    j -= k

为什么是这样连边？看不懂。