1 public class Solution {
 2     /**
 3      * @param matrix: a matrix of integers
 4      * @param k: an integer
 5      * @return: the kth smallest number in the matrix
 6      */
 7
 8         // write your code here
 9         public class Point {
10             public int x, y, val;
11             public Point(int x, int y, int val) {
12                 this.x = x;
13                 this.y = y;
14                 this.val = val;
15             }
16         }
17
18         Comparator<Point> comp = new Comparator<Point>() {
19             public int compare(Point left, Point right) {
20                 return left.val - right.val;
21             }
22         };
23
24         public int kthSmallest(int[][] matrix, int k) {
25             if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {
26                 return 0;
27             }
28             if (k > matrix.length * matrix[0].length) {
29                 return 0;
30             }
31             return horizontal(matrix, k);
32         }
33
34         private int horizontal(int[][] matrix, int k) {
35             Queue<Point> heap = new PriorityQueue<Point>(k, comp);
36             for (int i = 0; i < Math.min(matrix.length, k); i++) {
37                 heap.offer(new Point(i, 0, matrix[i][0]));
38             }
39             for (int i = 0; i < k - 1; i++) {
40                 Point curr = heap.poll();
41                 if (curr.y + 1 < matrix[0].length) {
42                     heap.offer(new Point(curr.x, curr.y + 1, matrix[curr.x][curr.y + 1]));
43                 }
44             }
45             return heap.peek().val;
46         }
47
48
49 }