决策树：

代码设计

代码：

class Solution {
    boolean[][] vis;
    int ret,m,n;

    public int getMaximumGold(int[][] grid) {
        m = grid.length;
        n = grid[0].length;
        vis = new boolean[m][n]; 

        for(int i = 0; i < m; i++)
            for(int j = 0; j < n; j++){
                //剪枝
                if (grid[i][j] != 0 && !vis[i][j]){
                    vis[i][j] = true;
                    dfs(grid,i,j,grid[i][j]);
                    vis[i][j] = false;//回溯
                }
            }

        return ret;    
    }

    int[] dx = {-1,1,0,0};
    int[] dy = {0,0,-1,1};
    //这里局部变量会自己恢复现场
    private void dfs(int[][] grid, int i, int j, int path){

        ret = Math.max(ret,path);
        
        for(int k = 0; k < 4; k++){
            int x = i + dx[k];
            int y = j + dy[k];
            //剪枝
            if(x >= 0 && x < m && y >= 0 && y < n && !vis[x][y] && grid[x][y] != 0){
                vis[x][y] = true;
                dfs(grid,x,y,path + grid[x][y]);
                vis[x][y] = false;//回溯
            }
        }
    }
}