867. Transpose Matrix 
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Problem
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.lengthn == matrix[i].length1 <= m, n <= 10001 <= m * n <= 105-109 <= matrix[i][j] <= 109