Number of Substrings With Only 1s

Description

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

Solution(javascript)

/**
 * @param {string} s
 * @return {number}
 */
const numSub = function (s) {
  const base = (10 ** 9) + 7
  let left = 0
  let count = 0
  for (let right = 0; right < s.length; right++) {
    const current = s[right]
    if (current === '1') {
      count = ((count % base) + (right - left + 1 % base)) % base // 以右边结尾的
    } else {
      left = right + 1
    }
  }
  return count
}