Number Of Substrings With Only 1s - Leetcode Solution

Problem:

The problem Number of Substrings With Only 1s asks us to count the number of contiguous substrings in a given binary string that contains only 1s.

For example, if the input binary string is "00110011", then the output should be 6, as there are six substrings with only 1s: "11", "111", "1111", "11", "11", and "1".

Approach:

We can solve this problem efficiently using the sliding window technique. The idea is to move a window of size k (where k is the length of the substring with only 1s) over the input string and count the number of valid substrings.

The steps for the sliding window approach are as follows:

1. Initialize two variables, start and end, to 0.

2. Set a variable count to 0, which will store the number of valid substrings.

3. Loop over the input string.

4. If a character is '1', increment the window end by 1.

5. If a character is '0', reset the window start to end and move the window end by 1.

6. Check if the current window contains only 1s. If yes, then increment the count by the length of the window.

7. After the loop, return the count.

Code:

Here's the Python code for the above algorithm:

class Solution: def numSub(self, s: str) -> int: start, end, count = 0, 0, 0 n = len(s) while end < n: if s[end] == '1': end += 1 elif s[end] == '0': start = end end += 1 if '1' * (end - start) == s[start:end]: count += end - start return count

Time Complexity:

The time complexity of the above algorithm is O(n), where n is the length of the input string.

Space Complexity:

The space complexity of the above algorithm is O(1), as we are not using any additional data structures to solve the problem.

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