LeetCode 2125 solution

2125. Number of Laser Beams in a Bank

# Medium # Array # Matrix

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.

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:
* bank[0][1] -- bank[2][1]
* bank[0][1] -- bank[2][3]
* bank[0][2] -- bank[2][1]
* bank[0][2] -- bank[2][3]
* bank[0][5] -- bank[2][1]
* bank[0][5] -- bank[2][3]
* bank[2][1] -- bank[3][2]
* bank[2][3] -- bank[3][2]
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.

Python

class Solution:
    def numberOfBeams(self, bank: List[str]) -> int:
        one_list = []
        result = []
        for b in bank:
            one = sum(1 for _ in b if _ == "1")
            if one == 0:
                continue
            else:
                one_list.append(one)
        if len(one_list) == 1:
            return 0
        for i in range(len(one_list) - 1):
            prev, next = one_list[i], one_list[i + 1]
            result.append(prev * next)
        return sum(result)

So simple algorithm it is. All you need to do is..

1. count "1" in for each array
2. if it's zero, skip that part
3. iterate the recorded count of 1 and multiply one by one and record the result int

that all.