binary_string.py

"""
Binary String

Given a binary string S. The task is to count the number of substrings that start and end with 1. For example, if the input string is “00100101”, then there are three substrings “1001”, “100101” and “101”.

Example 1:

Input:
N = 4
S = 1111
Output: 6
Explanation: There are 6 substrings from
the given string. They are 11, 11, 11,
111, 111, 1111.

Example 2:

Input:
N = 5
S = 01101
Output: 3
Explanation: There 3 substrings from the
given string. They are 11, 101, 1101.

Your Task:
The task is to complete the function binarySubstring() which takes the length of binary string and a binary string as input parameter and counts the number of substrings starting and ending with 1 and returns the count.

Expected Time Complexity: O(N).
Expected Auxiliary Space: O(1).

Constraints:
1 ≤ |S| ≤ 104
"""

import re
def count_total_1(string):
    findlist = re.findall('1', string)
    return len(findlist)

def binarySubstring(n,s):
    #code here
    tot_1 = count_total_1(s)
    ncr = (tot_1 * (tot_1 - 1))//2
    return ncr

if __name__ == "__main__":
    s = input()
    print(binarySubstring(len(s), s))