Number of Substrings With Only 1s

main.md

Question

Approach

I scan the string once and accumulate lengths of consecutive '1' runs. For each run of length L the number of substrings containing only '1' inside that run is L*(L+1)//2. I sum those values while taking modulo 10**9 + 7 to avoid overflow, and return the result.

Implementation

Complexities

• Time: O(n)
• Space: O(1)