Remember this pattern for all 2D array problems :
int rows = matrix.length and
int columns = matrix[0].length. Once you have the row / column dimensions (in this case, the same), write 2 for loops that traverse the top half of the square matrix. In the inner for loop, use a temporary variable to make the swap.
temp = matrix[i][
matrix[i][j] = matrix[j][
matrix[j][i] = temp;
