In a gold mine `grid` of size `m x n`, each cell in this mine has an integer representing the amount of gold in that cell, `0` if it is empty.

Return the maximum amount of gold you can collect under the conditions:

- Every time you are located in a cell you will collect all the gold in that cell.
- From your position, you can walk one step to the left, right, up, or down.
- You can't visit the same cell more than once.
- Never visit a cell with `0` gold.
- You can start and stop collecting gold from any position in the grid that has some gold.

Example 1:

    Input: grid = [[0,6,0],[5,8,7],[0,9,0]]
    Output: 24
    Explanation:
    [[0,6,0],
     [5,8,7],
     [0,9,0]]
    Path to get the maximum gold, 9 -> 8 -> 7.

Example 2:

    Input: grid = [[1,0,7],[2,0,6],[3,4,5],[0,3,0],[9,0,20]]
    Output: 28
    Explanation:
    [[1,0,7],
     [2,0,6],
     [3,4,5],
     [0,3,0],
     [9,0,20]]
    Path to get the maximum gold, 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7.

Constraints:

- `m == grid.length`
- `n == grid[i].length`
- `1 <= m, n <= 15`
- `0 <= grid[i][j] <= 100`
- There are at most 25 cells containing gold.