[Leetcode]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:
/*algorithm
select the top right element m,
compare the target with m,
if(target > m)--row
if(target < m)--colum
loop until target== m or not found
time O(max(m,n)),space O(1)
 */
bool searchMatrix(vector<vector<int>>& matrix, int target) {
int m = matrix.size();
if(m < 1)return false;
int n = matrix[0].size();
int i = 0,j = n - 1;
while(i < m && j > -1){
int val = matrix[i][j];
if(val == target)return true;
if(val < target)++i;
else --j;
}
return false;
}
};