LeetCode No.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.*/public class Solution {    public boolean searchMatrix(int[][] matrix, int target) {        if(matrix.length<1||matrix[0].length<1)            return false;        int a = matrix.length-1;        int b = 0;        while (a>=0&&b<matrix[0].length){//从左下角开始，比目标大则减一行，比目标小则加一列            if (matrix[a][b]==target)                return true;            if(matrix[a][b]>target){                a--;            }            else {                b++;            }        }        return false;    }}