【leetcode】1513. Number of Substrings With Only 1s

题目如下：

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

解题思路：对于所有字符都为1并且长度为n的字符串，满足题目条件的子串的个数为n*(n+1)/2。

代码如下：

class Solution(object):
    def numSub(self, s):
        """
        :type s: str
        :rtype: int
        """
        s += '# '
        count_1 = 0
        res = 0
        for i in s:
            if i == '1':
                count_1 += 1
            else:
                res += ((count_1)*(count_1+1)/2)
                count_1 = 0
        return res % (10**9 + 7)