417-Pacific-Atlantic-Water-Flow.java

class Solution {

    int[][] dir = { { 0, 1 }, { 0, -1 }, { 1, 0 }, { -1, 0 } };

    public List<List<Integer>> pacificAtlantic(int[][] heights) {
        List<List<Integer>> res = new ArrayList<>();

        int rows = heights.length, cols = heights[0].length;
        boolean[][] pacific = new boolean[rows][cols];
        boolean[][] atlantic = new boolean[rows][cols];

        for (int i = 0; i < cols; i++) {
            dfs(heights, 0, i, Integer.MIN_VALUE, pacific);
            dfs(heights, rows - 1, i, Integer.MIN_VALUE, atlantic);
        }

        for (int i = 0; i < rows; i++) {
            dfs(heights, i, 0, Integer.MIN_VALUE, pacific);
            dfs(heights, i, cols - 1, Integer.MIN_VALUE, atlantic);
        }

        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                if (pacific[i][j] && atlantic[i][j]) {
                    res.add(Arrays.asList(i, j));
                }
            }
        }
        return res;
    }

    private void dfs(
        int[][] heights,
        int i,
        int j,
        int prev,
        boolean[][] ocean
    ) {
        if (i < 0 || i >= ocean.length || j < 0 || j >= ocean[0].length) return;
        if (heights[i][j] < prev || ocean[i][j]) return;

        ocean[i][j] = true;
        for (int[] d : dir) {
            dfs(heights, i + d[0], j + d[1], heights[i][j], ocean);
        }
    }
}