[LeetCode] 417. Pacific Atlantic Water Flow

 轉自LeetCode

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

Solution

這題並不難，典型 DFS or BFS 的題目

唯一會需要想一下的地方是，題目要求的是可以同時流向 pacific 和 atlantic 的 cell

從上方和左方出去，和從右方和下方出去，是不同的

既然這樣，那就分開看，全部都是 pacific 和全部都是 atlantic

最後取交集，也就是 pacific[i][j] = true = atlantic[i][j] 的 cell 就是所求

另外一個要注意的地方，因為是要找可以流出去的

所以就直接從四個邊開始往內找，因此高度的判斷會反過來，下一個高必須比現在的高

kotlin