Leetcode 2125: Number of Laser Beams in a Bank

In this leetcode problem, we are counting the number of laser beams in a bank according to some rules.

Anti-theft security devices are activated inside a bank. You are given a 0-indexed binary string array bank representing the floor plan of the bank, which is an m x n 2D matrix. bank[i] represents the ith row, consisting of '0's and '1's. '0' means the cell is empty, while'1' means the cell has a security device.

There is one laser beam between any two security devices if both conditions are met:
The two devices are located on two different rows: r1 and r2, where r1 < r2.
For each row i where r1 < i < r2, there are no security devices in the ith row.
Laser beams are independent, i.e., one beam does not interfere nor join with another.
Return the total number of laser beams in the bank.

We are given the following example and solution:

Example

We see from the example and from the problem statement that we can simply ignore the rows without any laser, as they don’t change the results at all.

We can also observe that between a lever with X lasers and an adjacent level with Y lasers, the number of laser beams will be X*Y.

In summary, we only need to count the number of lasers at each level, and add the product between adjacent levels (ignoring empty…