LeetCode 240. Search a 2D Matrix II

问题描述：

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

.
思路：
因为是每行每列是顺序排列的，所以先按行从后往前搜索，再按列从后往前搜索就能减小复杂度

code：

class Solution {
public:
bool searchMatrix(vector<vector<int>>& matrix, int target) {
int h = matrix.size();

for(int i=h-1;i>=0;i--){
int w = matrix[0].size();
if(w==0)
continue;
if(matrix[i][0]<=target){
for(int j=w-1;j>=0;j--){
if(matrix[i][j]==target)
return true;
}
}
}
return false;

}
};