Leetcode: Search a 2D Matrix II

Write an efficient algorithm that searches for a value in an m x n matrix. This matrix has the following properties:

Integers in each row are sorted in ascending from left to right.
Integers in each column are sorted in ascending from top to bottom.
For example,

Consider the following matrix:

[
  [1,   4,  7, 11, 15],
  [2,   5,  8, 12, 19],
  [3,   6,  9, 16, 22],
  [10, 13, 14, 17, 24],
  [18, 21, 23, 26, 30]
]
Given target = 5, return true.

Given target = 20, return false.

 1 public class Solution {
 2     public boolean searchMatrix(int[][] matrix, int target) {
 3         int m = matrix.length;
 4         int n = matrix[0].length;
 5         if (matrix==null || m==0 || n==0) return false;
 6         int i=0, j=n-1;
 7         while (i<m && j>=0) {
 8             if (matrix[i][j] == target) return true;
 9             else if (matrix[i][j] > target) {
10                 j--;
11             }
12             else i++;
13         }
14         return false;
15     }
16 }