PacificAtlanticWaterFlow.java

class Solution {
    public List<List<Integer>> pacificAtlantic(int[][] heights) {
        int rows = heights.length, cols = heights[0].length;
        boolean[][] pac = new boolean[rows][cols];
        boolean[][] atl = new boolean[rows][cols];
        
        for (int col = 0; col< cols; col++){
            dfs(0, col, rows, cols, pac, heights[0][col], heights);
            dfs(rows-1, col,rows, cols, atl, heights[rows-1][col], heights);
        }
        for (int row = 0; row<rows; row++){
            dfs(row, 0,rows, cols, pac, heights[row][0], heights);
            dfs(row, cols-1,rows, cols, atl, heights[row][cols-1], heights);
        }
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        for (int i = 0; i < rows; i++)
            for (int j = 0; j < cols; j++){
                if (pac[i][j] && atl[i][j])
                    result.add(Arrays.asList(i,j));
            }
        return result;
    }
    private void dfs(int row, int col, int rows, int cols, boolean[][] visited, int prevHeight, int[][] heights){
        if (row < 0 || row >= rows || col < 0 || col >= cols || visited[row][col] || prevHeight > heights[row][col])
            return;
        visited[row][col]= true;
        dfs(row+1, col, rows, cols, visited, heights[row][col], heights);
        dfs(row-1, col, rows, cols, visited, heights[row][col], heights);
        dfs(row, col+1, rows, cols, visited, heights[row][col], heights);
        dfs(row, col-1, rows, cols, visited, heights[row][col], heights);
        
    }
}