240. Search a 2D Matrix II

//O(m+n) 220ms
class Solution {
public:
bool searchMatrix(vector<vector<int>>& m, int t) {
int a=m.size(),b=m[0].size();
if(a*b==0||m[0][0]>t||m[a-1][b-1]<t) return false;
int x=0,y=b-1;
while(x<a&&y>=0)
{
if(m[x][y]==t) return true;
if(m[x][y]<t) x++;
else y--;
}

return false;
}
};