240. 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.

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