1513. Number of Substrings With Only 1s

Problem Link

Description

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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 1:

Input: s = "0110111"
Output: 9
Explanation: There are 9 substring in total with only 1's characters.
"1" -> 5 times.
"11" -> 3 times.
"111" -> 1 time.

Example 2:

Input: s = "101"
Output: 2
Explanation: Substring "1" is shown 2 times in s.

Example 3:

Input: s = "111111"
Output: 21
Explanation: Each substring contains only 1's characters.

 

Constraints:

• 1 <= s.length <= 105
• s[i] is either '0' or '1'.

Solution

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Python3

class Solution:
    def numSub(self, S: str) -> int:
        
        M = int(1e9+7)
        count = res = 0
        for s in S:
            count = count + 1 if s == "1" else 0
            res += count
        
        return res%M