Given an m x n picture consisting of black 'B' and white 'W' pixels, return the number of black lonely pixels. A black lonely pixel is a character 'B' that located at a specific position where the same row and same column don't have any other black pixels. Example 1: Input: picture = [["W","W","B"],["W","B","W"],["B","W","W"]] Output: 3 Explanation: All the three 'B's are black lonely pixels. Example 2: Input: picture = [["B","B","B"],["B","B","B"],["B","B","B"]] Output: 0 Constraints: m == picture.length n == picture[i].length 1 <= m, n <= 500 picture[i][j] is 'W' or 'B'.