整体思路：

代码设计：

代码呈现：

class Solution {int ret,step;int m,n;boolean[][] vis;public int uniquePathsIII(int[][] grid) {m = grid.length;n = grid[0].length;vis = new boolean[m][n];int bx = 0;int by = 0;for(int i = 0; i < m; i++)for(int j = 0; j < n; j++){if(grid[i][j] == 0) step++;else if(grid[i][j] == 1){bx = i;by = j;}}step += 2;    vis[bx][by] = true;dfs(grid,bx,by,1);return ret;    }int[] dx = {0,0,-1,1};int[] dy = {-1,1,0,0};public void dfs(int[][] grid,int i,int j, int count){if(grid[i][j] == 2) {if(count == step) ret++;return;}for(int k = 0; k < 4; k++){int x = i + dx[k];int y = j + dy[k];if(x >= 0 && x < m && y >= 0 && y < n && !vis[x][y] && grid[x][y] != -1){vis[i][j] = true;dfs(grid,x,y,count+1);vis[x][y] = false;}}}
}