LeetCode 980. Unique Paths III

Problem Statement

impl Solution {
    fn dfs(grid: &mut Vec<Vec<i32>>, x: i32, y: i32, n: i32) -> i32 {
        if x < 0 || x == grid[0].len() as i32 || y < 0 || y == grid.len() as i32 ||
            grid[y as usize][x as usize] == -1 {
            return 0;
        }
        let (yy, xx) = (y as usize, x as usize);
        if grid[yy][xx] == 2 {
            if n == 0 {
                return 1;
            } else {
                return 0;
            }
        }
        grid[yy][xx] = -1;
        let ans = Self::dfs(grid, x + 1, y, n - 1) +
            Self::dfs(grid, x - 1, y, n - 1) +
            Self::dfs(grid, x, y + 1, n - 1) +
            Self::dfs(grid, x, y - 1, n - 1);
        grid[yy][xx] = 0;
        return ans;
    }
    pub fn unique_paths_iii(grid: Vec<Vec<i32>>) -> i32 {
        let (mut sx, mut sy, mut n) = (0, 0, 1);
        let mut g = grid.clone();
        for i in 0..g.len() {
            for j in 0..g[0].len() {
                if g[i][j] == 0 {
                    n += 1;
                } else if grid[i][j] == 1 {
                    sx = j; sy = i;
                }
            }
        }

        return Self::dfs(&mut g, sx as i32, sy as i32, n);
    }
}