LeetCode-in-Java
2466. Count Ways To Build Good Strings
Medium
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'zerotimes. - Append the character
'1'onetimes.
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.
Example 1:
Input: low = 3, high = 3, zero = 1, one = 1
Output: 8
Explanation:
One possible valid good string is “011”.
It can be constructed as follows: “” -> “0” -> “01” -> “011”.
All binary strings from “000” to “111” are good strings in this example.
Example 2:
Input: low = 2, high = 3, zero = 1, one = 2
Output: 5
Explanation: The good strings are “00”, “11”, “000”, “110”, and “011”.
Constraints:
1 <= low <= high <= 1051 <= zero, one <= low
Solution
public class Solution {
public int countGoodStrings(int low, int high, int zero, int one) {
int[] dp = new int[high + 1];
dp[zero]++;
dp[one]++;
int ans = 0;
for (int i = 0; i < high + 1; i++) {
if (dp[i] != 0) {
if (i + zero <= high) {
dp[i + zero] += dp[i];
dp[i + zero] = dp[i + zero] % 1000000007;
}
if (i + one <= high) {
dp[i + one] += dp[i];
dp[i + one] = dp[i + one] % 1000000007;
}
if (i >= low) {
ans += dp[i];
ans = ans % 1000000007;
}
}
}
return ans;
}
}