Lonely Pixel I

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","W"],
                 ["B","B","B"]]

Output: 0

Constraints:

• m == picture.length
• n == picture[i].length
• 1 <= m, n <= 500
• picture[i][j] is 'W' or 'B'.

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