1513. Number of Substrings With Only 1s

https://leetcode.com/problems/number-of-substrings-with-only-1s/

Given a binary string s (a string consisting only of '0' and '1's).

Return the number of substrings with all characters 1's.

Since the answer may be too large, return it modulo 10^9 + 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.

Example 4:

Input: s = "000"
Output: 0

Constraints:

• s[i] == '0' or s[i] == '1'

• 1 <= s.length <= 10^5

二进制数组中有多少个由1组成的子数组。subarray => 窗口or DP。遍历，以i为底长度为l的全1数组能通过从i往前扩展生成l个新子数组。

class Solution:
    def numSub(self, s: str) -> int:
        dp, res, MOD = 0, 0, 10 ** 9 + 7
        for c in s:
            dp = 0 if c == '0' else dp + 1
            res = (res + dp) % MOD
        return res