力扣-980-不同路径III

public int uniquePathsIII(int[][] grid) {
    if(grid == null)    return 0;
    int startY = 0,startX = 0,stepNum = 1;

    for(int i = 0;i < grid.length;i++){
        for(int j = 0;j<grid[0].length;j++){
            if(grid[i][j] == 1){
                startY = i;
                startX = j;
                continue;
            }
            if(grid[i][j] == 0) stepNum++;//每个无障碍方格都要通过一次
        }
    }
    return dfs(startX,startY,stepNum,grid);
}

private int dfs(int x,int y,int stepSur,int [][] grid){
    if(x < 0 || x >= grid[0].length || y < 0 || y >= grid.length || grid[y][x] == -1)   return 0;
    if(grid[y][x] == 2) return stepSur == 0 ? 1 : 0;//无障碍方格走完没，走完+1,没走完+0不算
    grid[y][x] = -1;//走过的标记设为-1
    int res = 0;
    res += dfs(x-1,y,stepSur - 1,grid);
    res += dfs(x+1,y,stepSur - 1,grid);
    res += dfs(x,y-1,stepSur - 1,grid);
    res += dfs(x,y+1,stepSur - 1,grid);
    grid[y][x] = 0;

    return res;
}