2125.go

// Source: https://leetcode.com/problems/number-of-laser-beams-in-a-bank
// Title: Number of Laser Beams in a Bank
// Difficulty: Medium
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// 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:
//
//   https://assets.leetcode.com/uploads/2021/12/24/laser1.jpg
//
//   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 0th row with any on the 3rd row.
//     This is because the 2nd row contains security devices, which breaks the second condition.
//
// Example 2:
//
//   https://assets.leetcode.com/uploads/2021/12/24/laser2.jpg
//
//   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'.
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

package main

import (
	"strings"
)

// O(n) time, O(1) space
// Sum of products of devices count between adjency rows. Skipping empty rows
func numberOfBeams(bank []string) int {
	res := 0
	prevCount := 0
	for _, row := range bank {
		currCount := strings.Count(row, "1")
		if currCount == 0 {
			continue
		}
		res += prevCount * currCount
		prevCount = currCount
	}
	return res
}