Given a binary string s, return the number of substrings with all characters1'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;
}
}