search_a_2d_matrix_2

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        int m = matrix.size();
        if(m==0) return false;
        int n = matrix[0].size();
        // the upper-right element is the special one: 
        //all elements less than it are at its left side and all elements greater than it are behind.
        int i=0, j=n-1;
        while(i<m&&j>=0){
            if(matrix[i][j]<target){
                i++;
            }else if(matrix[i][j]>target){
                j--;
            }else{
                return true;
            }
        }
        return false;
    }
};