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.

public class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        if(matrix.length==0 || matrix[0].length==0) 
            return false;  
          
        int i=matrix.length-1, j=0;  
          
        while(i>=0 && j<matrix[0].length) {  
            int x = matrix[i][j];  
            if(target == x) 
                return true;  
            else if(target < x) 
                --i;  
            else ++j;  
        }  
        return false;  
    }
}