顺丰笔试： 幸运数

n = 21857711

nums = str(n)

length = len(nums)
count = 0

i = 0

#统计n位数中满足情况的个数，逐位遍历
for char in nums:
    num = int(char)
    i += 1
    if num < 4:
        break
    if num > 4 and num < 7:
        count += pow(2, (length - i))
        break
    if num > 7:
        count += pow(2, (length - i + 1))
        break
    if num is 7:
        count += pow(2, (length - i))

    if i is length:
        count += 1

ans = count % (pow(10, 9)+7)
#统计小于n位的
for j in range(1, length):
    ans += (pow(2, j) % (pow(10, 9)+7))

print(ans % (pow(10, 9)+7))