417. Pacific Atlantic Water Flow

func pacificAtlantic(_ matrix: [[Int]]) -> [[Int]] {
        guard matrix.count > 0 && matrix[0].count > 0  else { return [] }
        let n = matrix.count
        let m = matrix[0].count

        var dpleft = [[Bool]](repeating: [Bool](repeating : false, count : m), count: n)
        var dpright = [[Bool]](repeating: [Bool](repeating : false, count : m), count: n)
        var res = [[Int]]()
        for i in 0 ..< n {
            fillWater(matrix, &dpleft, i, 0)
            fillWater(matrix, &dpright, i, m - 1)
        }
        for j in 0 ..< m {
            fillWater(matrix, &dpleft, 0, j)
            fillWater(matrix, &dpright, n - 1, j)
        }
        for i in 0 ..< n {
            for j in 0 ..< m{
                if dpleft[i][j] && dpright[i][j]{
                    res.append([i, j])
                }
            }
        }
        return res

    }

    let dx = [1, -1 ,0 ,0 ]
    let dy = [0 ,0 ,1 ,-1 ]

    func fillWater(_ m: [[Int]], _ check: inout [[Bool]], _ i:Int, _ j:Int){
        check[i][j] = true
        for k in 0 ..< 4 {
            let nx = dx[k] + i
            let ny = dy[k] + j
            if nx >= 0 && ny >= 0 && nx < m.count && ny < m[0].count && m[nx][ny] >= m[i][j] && !check[nx][ny] {
                fillWater(m, &check, nx, ny)
            }
        }
    }