2125. Number of Laser Beams in a Bank
Approach
For each row count devices; multiply consecutive non-zero row counts.
Key Techniques
Array problems involve manipulating elements stored in a contiguous block of memory. Key techniques include two-pointer traversal, prefix sums, sliding windows, and in-place partitioning. In C#, arrays are zero-indexed and fixed in size — use List<T> when you need dynamic resizing.
Math problems test number theory, combinatorics, and modular arithmetic. Common tools: GCD/LCM (Euclidean algorithm), prime sieve, modular inverse (Fermat's little theorem), digit manipulation, and bit tricks. Overflow is a key concern in C# — use long when products may exceed 2³¹.
Matrix problems often involve BFS/DFS flood fill, dynamic programming on 2D grids, or spiral/diagonal traversal. For row × column DP, break it into 1D sub-problems column by column. Common pitfalls: boundary checks and modifying the input matrix in-place.
// Approach: For each row count devices; multiply consecutive non-zero row counts.
// Time: O(mn) Space: O(1)
public class Solution
{
public int NumberOfBeams(string[] bank)
{
int laserBeam = 0;
int n = bank.Length;
int prevBankDeviceCnt = 0;
for (int i = 0; i < bank.Length; i++)
{
string b = bank[i];
int deviceCnt = 0;
int j = 0;
while (j < b.Length)
{
if (b[j] == '1')
deviceCnt++;
j++;
}
if (deviceCnt > 0)
{
laserBeam += prevBankDeviceCnt * deviceCnt;
prevBankDeviceCnt = deviceCnt;
}
}
return laserBeam;
}
}