3092. Minimum Moves to Spread Stones Over Grid

Medium
Array
Dynamic Programming
Breadth-First Search
Matrix

Description

You are given a 0-indexed 2D integer matrix grid of size 3 * 3, representing the number of stones in each cell. The grid contains exactly 9 stones, and there can be multiple stones in a single cell.

In one move, you can move a single stone from its current cell to any other cell if the two cells share a side.

Return the minimum number of moves required to place one stone in each cell.

 

Example 1:

Input: grid = [[1,1,0],[1,1,1],[1,2,1]]
Output: 3
Explanation: One possible sequence of moves to place one stone in each cell is: 
1- Move one stone from cell (2,1) to cell (2,2).
2- Move one stone from cell (2,2) to cell (1,2).
3- Move one stone from cell (1,2) to cell (0,2).
In total, it takes 3 moves to place one stone in each cell of the grid.
It can be shown that 3 is the minimum number of moves required to place one stone in each cell.

Example 2:

Input: grid = [[1,3,0],[1,0,0],[1,0,3]]
Output: 4
Explanation: One possible sequence of moves to place one stone in each cell is:
1- Move one stone from cell (0,1) to cell (0,2).
2- Move one stone from cell (0,1) to cell (1,1).
3- Move one stone from cell (2,2) to cell (1,2).
4- Move one stone from cell (2,2) to cell (2,1).
In total, it takes 4 moves to place one stone in each cell of the grid.
It can be shown that 4 is the minimum number of moves required to place one stone in each cell.

 

Constraints:

  • grid.length == grid[i].length == 3
  • 0 <= grid[i][j] <= 9
  • Sum of grid is equal to 9.

Hints

Hint 1
There are at most <code>4</code> cells with more than one stone.
Hint 2
Let <code>a</code> be the number of cells containing more than one stone, and <code>b</code> be the number of cells containing no stones. <code></code>. <code>b^a ≤ 6561</code>. Use this fact to come up with a bruteforce.
Hint 3
For all empty cells, bruteforce over all possible cells from which a stone can come. Note that a stone will always come from a cell containing at least 2 stones.

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