In a gold mine grid of size m * 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:
<li>Every time you are located in a cell you will collect all the gold in that cell.</li>
<li>From your position you can walk one step to the left, right, up or down.</li>
<li>You can't visit the same cell more than once.</li>
<li>Never visit a cell with <code>0</code> gold.</li>
<li>You can start and stop collecting gold from <strong>any </strong>position in the grid that has some gold.</li>
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:
<li><code>1 <= grid.length, grid[i].length <= 15</code></li>
<li><code>0 <= grid[i][j] <= 100</code></li>
<li>There are at most <strong>25 </strong>cells containing gold.</li>