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

.

思路：从左下角开始search，target < value, 往上移动，target > value, 往右移动。

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