Search a 2D Matrix II

题目描述：

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

.
解题思路：
从右上角row=0,col=cols-1开始查找target元素，如果matrix[row][col]=target返回true，如果matrix[row][col]>target，则令col--；如果matrix[row][col]<target，则令row++

AC代码如下：

class Solution {
public:
bool searchMatrix(vector<vector<int>>& matrix, int target) {
int rows = matrix.size();
if (rows == 0 || matrix[0].size() == 0) return false;
int cols = matrix[0].size();
int row = 0;
int col = cols - 1;
while (row < rows && col >= 0){
int cur_num = matrix[row][col];
if (cur_num == target) return true;
else if (cur_num > target) col--;
else row++;
}
return false;
}
};