Given an `m x n` binary matrix `mat`, return the number of special positions in `mat`.

A position `(i, j)` is called special if `mat[i][j] == 1` and all other elements in row `i` and column `j` are `0` (rows and columns are 0-indexed).

Example 1:

    Input: mat = [[1,0,0],[0,0,1],[1,0,0]]
    Output: 1
    Explanation: (1, 2) is a special position because mat[1][2] == 1 and all other elements in row 1 and column 2 are 0.

Example 2:

    Input: mat = [[1,0,0],[0,1,0],[0,0,1]]
    Output: 3
    Explanation: (0, 0), (1, 1) and (2, 2) are special positions.

Constraints:

- `m == mat.length`
- `n == mat[i].length`
- `1 <= m, n <= 100`
- `mat[i][j]` is either `0` or `1`.