2125 - Number of Laser Beams in a Bank.

Medium

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 <code>i^th</code> 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: <code>r₁</code> and <code>r₂</code>, where <code>r₁< r₂</code>.

• For each row i where <code>r₁< i < r₂</code>, there are no security devices in the <code>i^th</code> 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.

Example 1:

Input: bank = "011001","000000","010100","001000"

Output: 8

Explanation: Between each of the following device pairs, there is one beam. In total, there are 8 beams:

• bank1 -- bank1

• bank1 -- bank3

• bank2 -- bank1

• bank2 -- bank3

• bank5 -- bank1

• bank5 -- bank3

• bank1 -- bank2

• bank3 -- bank2

Note that there is no beam between any device on the 0^th row with any on the 3^rd row.

This is because the 2^nd row contains security devices, which breaks the second condition.

Example 2:

Input: bank = "000","111","000"

Output: 0

Explanation: There does not exist two devices located on two different rows.

Constraints:

• m == bank.length

• n == bank[i].length

• 1 <= m, n <= 500

• bank[i][j] is either '0' or '1'.