Number of Substrings With Only 1s — LeetCode 1513 Python Solution

The problem

Given a binary string s, return the number of substrings with all characters 1's. Since the answer may be too large, return it modulo 109 + 7.

Example

Input

s = "0110111"

Output

9

Explanation

There are 9 substring in total with only 1's characters.

Python solution

class Solution:
    def numSub(self, s: str) -> int:
        mod = 10**9 + 7
        ans = cur = 0
        for c in s:
            if c == "0":
                cur = 0
            else:
                cur += 1
                ans = (ans + cur) % mod
        return ans

Complexity

Pattern: Math and Number Theory

Find the closed form, the invariant, or the modular identity — and skip the loop entirely. LeetCode 1513. Number of Substrings With Only 1s is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Math.

The math and number theory guide has the Python template for the pattern and the 485 LeetCode problems that use it.

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