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.

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.

从右上开始，该值小于target的话这一行都不要 row++，该值大于target的话这一列都不要 col--
class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        if(matrix == null || matrix.length == 0) return false;
        if(matrix[0] == null || matrix[0].length == 0) return false;
        int row = 0;
        int col = matrix[0].length - 1;
        while(row < matrix.length && col >= 0) {
            if(matrix[row][col] == target) return true;
            else if(matrix[row][col] < target) row++;
            else col--;
        }
        return false;
    }
}