This question is similar to bomb the enemy.

```
class Solution {
public:
int findLonelyPixel(vector<vector<char>>& picture) {
if (picture.empty() || picture[0].empty())
return 0;
int m = picture.size();
int n = picture[0].size();
int ans = 0;
for (int i = 0; i < m; ++i) {
int col = -1; // which col has the single black pixel in row i
for (int j = 0; j < n; ++j) {
if (picture[i][j] == 'B') {
if (col >= 0) {
col = -1;
break;
}
else col = j;
}
}
if (col >= 0) {
int count = 0;
for (int k = 0; k < m; ++k) {
if (picture[k][col] == 'B')
if (count++ == 1) break;
}
if (count == 1)
ans++;
}
}
return ans;
}
};
```