0417. Pacific Atlantic Water Flow

https://leetcode.com/problems/pacific-atlantic-water-flow

Description

There is an m x n rectangular island that borders both the Pacific Ocean and Atlantic Ocean. The Pacific Ocean touches the island's left and top edges, and the Atlantic Ocean touches the island's right and bottom edges.

The island is partitioned into a grid of square cells. You are given an m x n integer matrix heights where heights[r][c] represents the height above sea level of the cell at coordinate (r, c).

The island receives a lot of rain, and the rain water can flow to neighboring cells directly north, south, east, and west if the neighboring cell's height is less than or equal to the current cell's height. Water can flow from any cell adjacent to an ocean into the ocean.

Return a 2D list of grid coordinates result where result[i] = [ri, ci] denotes that rain water can flow from cell (ri, ci) to both the Pacific and Atlantic oceans.

Example 1:

**Input:** heights = [[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]]
**Output:** [[0,4],[1,3],[1,4],[2,2],[3,0],[3,1],[4,0]]

Example 2:

**Input:** heights = [[2,1],[1,2]]
**Output:** [[0,0],[0,1],[1,0],[1,1]]

Constraints:

• m == heights.length

• n == heights[r].length

• 1 <= m, n <= 200

• 0 <= heights[r][c] <= 105

ac1: DFS

reverse thinking

class Solution {
    public List<int[]> pacificAtlantic(int[][] matrix) {
        List<int[]> res = new ArrayList<int[]>();
        // edge cases
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {
            return res;
        }

        int row = matrix.length;
        int col = matrix[0].length;
        // two note matrix
        boolean[][] pacific = new boolean[row][col];
        boolean[][] atlantic = new boolean[row][col];

        // start from borders 
        int h = Integer.MIN_VALUE;
        for (int r = 0; r < row; r++) {
            dfs(matrix, pacific, r, 0, h);  
            dfs(matrix, atlantic, r, col-1, h);
        }
        for (int c = 0; c < col; c++) {
            dfs(matrix, pacific, 0, c, h);
            dfs(matrix, atlantic, row-1, c, h);
        }

        // get result
        for (int r = 0; r < row; r++) {
            for (int c = 0; c < col; c++) {
                if (pacific[r][c] && atlantic[r][c]) {
                    res.add(new int[]{r, c});
                }
            }
        }

        return res;
    }
    private void dfs(int[][] matrix, boolean[][] visited, int r, int c, int h) {
        if (r < 0 || r >= matrix.length || c < 0 || c >= matrix[0].length) return; // out of boundary
        if (visited[r][c]) return;  // visited before
        if (matrix[r][c] < h) return; // not qualified

        visited[r][c] = true;

        int[][] dirs = new int[][]{{-1, 0}, {1, 0}, {0, 1}, {0, -1}};
        for (int[] d : dirs) {
            dfs(matrix, visited, r+d[0], c+d[1], matrix[r][c]);
        }
    }    
}

/*
two matrix: P A
start from border, DFS
*/