1513. Number of Substrings With Only 1s

package LeetCode_1513

/**
 * 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:
1. s[i] == '0' or s[i] == '1'
2. 1 <= s.length <= 10^5
 * */
class Solution {
    /*
    * solution: keep counting the consecutive 1 then update countOne and result, but reset countOne=0 when encounter 0
    * Time:O(n), Space:O(1)
    * */
    fun numSub(s: String): Int {
        var result = 0
        var countOne = 0
        val mod = 1000000007
        for (c in s) {
            if (c == '1') {
                countOne++
            } else {
                countOne = 0
            }
            result = (result + countOne) % mod
        }
        return result
    }
}