Introduction

In this blog post, we will explore how to solve the LeetCode problem "2999" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like Chatmagic, we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

Problem Statement

You are given three integers start, finish, and limit. You are also given a 0-indexed string s representing a positive integer. A positive integer x is called powerful if it ends with s (in other words, s is a suffix of x) and each digit in x is at most limit. Return the total number of powerful integers in the range [start..finish]. A string x is a suffix of a string y if and only if x is a substring of y that starts from some index (including 0) in y and extends to the index y.length - 1. For example, 25 is a suffix of 5125 whereas 512 is not. Example 1: Input: start = 1, finish = 6000, limit = 4, s = "124" Output: 5 Explanation: The powerful integers in the range [1..6000] are 124, 1124, 2124, 3124, and, 4124. All these integers have each digit <= 4, and "124" as a suffix. Note that 5124 is not a powerful integer because the first digit is 5 which is greater than 4. It can be shown that there are only 5 powerful integers in this range. Example 2: Input: start = 15, finish = 215, limit = 6, s = "10" Output: 2 Explanation: The powerful integers in the range [15..215] are 110 and 210. All these integers have each digit <= 6, and "10" as a suffix. It can be shown that there are only 2 powerful integers in this range. Example 3: Input: start = 1000, finish = 2000, limit = 4, s = "3000" Output: 0 Explanation: All integers in the range [1000..2000] are smaller than 3000, hence "3000" cannot be a suffix of any integer in this range. Constraints: 1 <= start <= finish <= 1015 1 <= limit <= 9 1 <= s.length <= floor(log10(finish)) + 1 s only consists of numeric digits which are at most limit. s does not have leading zeros.

Explanation

Here's a breakdown of the solution:

• Determine the Range of Possible Prefixes: Calculate the minimum and maximum possible numbers with s as a suffix that fall within the [start, finish] range and whose digits are all less than or equal to limit.
• Count Valid Prefixes: Iterate through the possible lengths of the prefix and for each length, determine the range of numbers that could form a valid prefix. A prefix is valid if appending it to s results in a number within the [start, finish] range and all digits are less than or equal to limit.

• Efficient Calculation: Optimize the counting process by calculating the number of valid prefixes directly without explicitly generating and checking each one, using the digit constraint efficiently.

• Runtime Complexity: O(log(finish)), where finish is the upper bound of the given range.

• Storage Complexity: O(1)

Code

    def solve():
    start, finish, limit, s = map(eval, [input(), input(), input(), input()])

    def count_powerful(num):
        s_len = len(s)
        num_str = str(num)
        num_len = len(num_str)

        if s_len > num_len:
            return 0

        s_val = int(s)

        if s_val > num:
            return 0

        count = 0
        for prefix_len in range(num_len - s_len + 1):
            min_prefix_val = 0
            max_prefix_val = 0

            if prefix_len > 0:
                min_powerful = int('1' + '0' * (prefix_len - 1) + s)
                max_powerful = int('9' * prefix_len + s)

                if min_powerful > num:
                    continue

                prefix_min = 0
                if min_powerful >= start:
                    prefix_min = int(str(min_powerful)[:-s_len])

                prefix_max = float('inf')
                if max_powerful <= num:
                    prefix_max = int(str(max_powerful)[:-s_len])

                temp_max = int('0' + '9' * (prefix_len -1))

                if prefix_max > temp_max:
                    prefix_max = temp_max

                min_prefix_val = prefix_min
                max_prefix_val = prefix_max
            else:

                if s_val >= start and s_val <= num:
                    min_prefix_val = 0
                    max_prefix_val = 0
                else:
                    continue


            valid_min = 0
            if prefix_len > 0 :
                valid_min = min_prefix_val

            valid_max = max_prefix_val



            if prefix_len > 0:

                temp_valid_min = 0
                power_of_10 = 1
                for _ in range(prefix_len -1):
                    power_of_10 *= 10

                if start > int(str(valid_min).zfill(prefix_len) + s):
                    temp_valid_min = (start - int(s)) // (power_of_10 * (10 ** len(s)))
                    valid_min = temp_valid_min

                    while int(str(valid_min).zfill(prefix_len)+ s) < start:
                        valid_min +=1


            valid_count = 0
            if prefix_len > 0:
                max_possible_digit = limit
                temp_valid_max = 0

                for i in range(prefix_len):

                   temp_valid_max = temp_valid_max * 10 + max_possible_digit

                if valid_max > temp_valid_max:
                    valid_max = temp_valid_max


            if int(s) >= start and int(s) <= num and len(str(int(s))) == len(str(num)):
                all_digits_valid = True
                for digit in str(int(s)):
                    if int(digit) > limit:
                        all_digits_valid = False
                if all_digits_valid:
                    count+=1

            if prefix_len ==0 and int(s) > num:
                count +=0

            elif prefix_len > 0:

                all_digits_valid = True
                temp_range_count = valid_max - valid_min +1
                for i in range(prefix_len):
                    if limit < 0:
                        all_digits_valid = False
                        temp_range_count = 0
                if all_digits_valid and temp_range_count >=0:
                    valid = True
                    for digit in str(int(s)):
                        if int(digit) > limit:
                            valid = False
                            temp_range_count = 0

                    if valid:
                        count += temp_range_count

        return count

    result = count_powerful(finish) - count_powerful(start - 1)
    print(result)

solve()