The idea is to keep track of number of black pixels in each row and column.

```
public class Solution {
public int findLonelyPixel(char[][] picture) {
int n = picture.length;
int m = picture[0].length;
int[] rCount = new int[n];
int[] cCount = new int[m];
for(int i = 0; i < n; ++i)
{
for(int j=0; j < m; ++j)
{
if(picture[i][j]=='B')
{
++rCount[i];
++cCount[j];
}
}
}
int ans = 0;
for(int i = 0; i < n; ++i)
{
if(rCount[i]==1)
{
for(int j=0; j < m; ++j)
{
if(cCount[j] == 1 &&picture[i][j]=='B')
{
++ans;
}
}
}
}
return ans;
}
}
```