Given a 2D integer array `matrix`, return the transpose of `matrix`.

The transpose of a matrix is the matrix flipped over its main diagonal, switching the matrix's row and column indices.

Example 1:

    Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
    Output: [[1,4,7],[2,5,8],[3,6,9]]

Example 2:

    Input: matrix = [[1,2,3],[4,5,6]]
    Output: [[1,4],[2,5],[3,6]]

Constraints:

- `m == matrix.length`
- `n == matrix[i].length`
- `1 <= m, n <= 1000`
- `1 <= m * n <= 105`
- `-109 <= matrix[i][j] <= 109`