You cannot solve it without using a visited set/matrix. If you override a cell's value in a grid, then the algorithm can tell if this cell has been visited before, but it cannot tell if this cell had the same value as the current value of interest or if it had a differe...

You can never solve the problem without "visited cells" because you can't destroy the input.

In a matrix with lowercase letters, you could probably do it by marking a cell as visited by uppercasing the letter, and then iterate through the matrix and reset the characters back to lowercase before returning the result. Still would have the same time overall comple...