DES-3rd

length = []

stack = [length[0]]

curr_end = length[0]

n = len(length)
ans = n

for i in range(1, len(length)):
    if curr_end < length[i] +i:
        last = None
        while len(stack) > 1 and i < stack[-1]:
            last = stack[-1]
            stack.pop(-1)
        stack += [last, length[i]+i]
        curr_end = length[i]+i
        if curr_end >= n:
            ans = min(ans, len(stack))