Number of Substrings With Only 1s

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'.

Answer

class Solution {
    long mod = 1_000_000_007;

    public int numSub(String s) {
        long ans = 0;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '1') {
                // Find continous length of "1"s
                long len = 0;
                while (i < s.length() && s.charAt(i) == '1') {
                    i++;
                    len++;
                }
                i--;
                ans = (ans + ((len * (len + 1) % mod) % mod) / 2) % mod;
            }
        }
        return (int) ans;
    }
}