2466. Count Ways To Build Good Strings.py

"""
2466
Given the integers zero, one, low, and high, we can construct a string by starting with an empty string, and then at each step perform either of the following:

Append the character '0' zero times.
Append the character '1' one times.
This can be performed any number of times.

A good string is a string constructed by the above process having a length between low and high (inclusive).

Return the number of different good strings that can be constructed satisfying these properties. Since the answer can be large, return it modulo 109 + 7.

 

"""
class Solution:
    def countGoodStrings(self, low: int, high: int, zero: int, one: int) -> int:
        dp = [0] * (high + 1)
        dp[0] = 1
        md = 10 ** 9 + 7
        for i in range(min(zero, one), high + 1):
            if i >= zero:
                dp[i] = (dp[i] + dp[i - zero]) % md
            if i >= one:
                dp[i] = (dp[i] + dp[i - one]) % md

        s = 0
        for i in range(low, high + 1):
            s = (s + dp[i]) % md
        return s