417. Pacific Atlantic Water Flow

Given anm x nmatrix of non-negative integers representing the height of each unit cell in a continent, the "Pacific ocean" touches the left and top edges of the matrix and the "Atlantic ocean" touches the right and bottom edges.

Water can only flow in four directions (up, down, left, or right) from a cell to another one with height equal or lower.

Find the list of grid coordinates where water can flow to both the Pacific and Atlantic ocean.

Note:

1. The order of returned grid coordinates does not matter.
2. Both m and n are less than 150.

Example:

Given the following 5x5 matrix:

  Pacific ~   ~   ~   ~   ~ 
       ~  1   2   2   3  (5) *
       ~  3   2   3  (4) (4) *
       ~  2   4  (5)  3   1  *
       ~ (6) (7)  1   4   5  *
       ~ (5)  1   1   2   4  *
          *   *   *   *   * Atlantic

Return:

[[0, 4], [1, 3], [1, 4], [2, 2], [3, 0], [3, 1], [4, 0]] (positions with parentheses in above matrix).

class Coordinate {
    int x;
    int y;
    public Coordinate(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

public class Solution {
    private int n;
    private int m;
    private int[] dirX = {0, 0, 1, -1};
    private int[] dirY = {1, -1, 0, 0};
    public List<int[]> pacificAtlantic(int[][] matrix) {
        List<int[]> results = new ArrayList<>();
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {
            return results;
        }
        this.n = matrix.length;
        this.m = matrix[0].length;
        boolean[][] pGrid = new boolean[this.n][this.m];
        boolean[][] aGrid = new boolean[this.n][this.m];
        for (int i = 0; i < this.m; i++) {
            Coordinate pStart = new Coordinate(0, i);
            markByBFS(matrix, pStart, pGrid); 
            Coordinate aStart = new Coordinate(this.n - 1, i);
            markByBFS(matrix, aStart, aGrid);
        }
        for (int i = 0; i < this.n; i++) {
            Coordinate pStart = new Coordinate(i, 0);
            markByBFS(matrix, pStart, pGrid);
            Coordinate aStart = new Coordinate(i, this.m - 1);
            markByBFS(matrix, aStart, aGrid);
        }
        for (int i = 0; i < this.n; i++) {
            for (int j = 0; j < this.m; j++) {
                if (pGrid[i][j] && aGrid[i][j]) {
                    int[] point = new int[2];
                    point[0] = i;
                    point[1] = j;
                    results.add(point);
                }
            }
        }
        return results;
    }

    // BFS solution
    private void markByBFS(int[][] matrix, Coordinate start, boolean[][] grid) {
        boolean[][] visited = new boolean[this.n][this.m];
        Queue<Coordinate> queue = new LinkedList<Coordinate>();
        queue.offer(start);
        visited[start.x][start.y] = true;
        grid[start.x][start.y] = true;
        while (!queue.isEmpty()) {
            Coordinate head = queue.poll();
            for (int dir = 0; dir < 4; dir++) {
                int neiX = head.x + dirX[dir];
                int neiY = head.y + dirY[dir];
                if (inBound(neiX, neiY) && !visited[neiX][neiY] && matrix[head.x][head.y] <= matrix[neiX][neiY]) {
                    Coordinate nei = new Coordinate(neiX, neiY);
                    queue.offer(nei);
                    visited[neiX][neiY] = true;
                    grid[neiX][neiY] = true;
                }
            }
        }
    }

    // DFS solution
    private void markByDFS(int[][] matrix, Coordinate cor, boolean[][] grid) {
        grid[cor.x][cor.y] = true;
        for (int dir = 0; dir < 4; dir++) {
            int neiX = cor.x + dirX[dir];
            int neiY = cor.y + dirY[dir];
            if (inBound(neiX, neiY) && grid[neiX][neiY] == false && matrix[cor.x][cor.y] <= matrix[neiX][neiY]) {
                markByDFS(matrix, new Coordinate(neiX, neiY), grid);
            }
        }
    }

    private boolean inBound(int x, int y) {
        return x >= 0 && x < this.n && y >= 0 && y < this.m;
    }
}