2125. Number of Laser Beams in a Bank - Explanation

1. Counting

class Solution:
    def numberOfBeams(self, bank: List[str]) -> int:
        prev = bank[0].count("1")
        res = 0

        for i in range(1, len(bank)):
            curr = bank[i].count("1")
            if curr:
                res += prev * curr
                prev = curr

        return res

Time & Space Complexity

Time complexity: $O(m * n)$
Space complexity: $O(1)$ extra space.

Where $m$ is the number of rows and $n$ is the number of columns.