java面试题（7）

原题：

// Given an m x n matrix 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:    // The order of returned grid coordinates does not matter.    // 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).

答案：

public class Solution {    public List<int[]> pacificAtlantic(int[][] matrix) {        List<int[]> result = new LinkedList<>();        //error checking        if(matrix == null || matrix.length == 0 || matrix[0].length == 0) {            return result;        }        int n = matrix.length;        int m = matrix[0].length;        boolean[][] pacific = new boolean[n][m];        boolean[][] atlantic = new boolean[n][m];        for(int i = 0; i < n; i++) {            dfs(matrix, pacific, Integer.MIN_VALUE, i, 0);            dfs(matrix, atlantic, Integer.MIN_VALUE, i, m - 1);        }        for(int i = 0; i < m; i++) {            dfs(matrix, pacific, Integer.MIN_VALUE, 0, i);            dfs(matrix, atlantic, Integer.MIN_VALUE, n - 1, i);        }        for(int i = 0; i < n; i++) {            for(int j = 0; j < m; j++) {                if(pacific[i][j] && atlantic[i][j]) {                    result.add(new int[] {i, j});                }            }        }        return result;    }    public void dfs(int[][] matrix, boolean[][] visited, int height, int x, int y) {        int n = matrix.length;        int m = matrix[0].length;        if(x < 0 || x >= n || y < 0 || y >= m || visited[x][y] || matrix[x][y] < height) {            return;        }        visited[x][y] = true;        dfs(matrix, visited, matrix[x][y], x + 1, y);        dfs(matrix, visited, matrix[x][y], x - 1, y);        dfs(matrix, visited, matrix[x][y], x, y + 1);        dfs(matrix, visited, matrix[x][y], x, y - 1);    }}