題目：Write an efficient algorithm that searches for a value in an mn matrix. This matrix has the following
properties:
• Integers in each row are sorted from left to right.
• The first integer of each row is greater than the last integer of the previous row.
For example, Consider the following matrix:
[
[1, 3, 5, 7],
[10, 11, 16, 20],
[23, 30, 34, 50]
]
Given target = 3, return true.

可以用二分查詢解決，程式碼如下：

// 時間複雜度 O(log(nm))，空間複雜度 O(1)
class Solution {
public:
        bool searchMatrix(vector<vector<int>>& matrix, int target) {
                if (matrix.empty()) return false;
                const size_t m = matrix.size();
                const size_t n = matrix[0].size();

                int
 
 first = 0;
                int last = m * n;
                while (first != last) {
                        int mid = first + (last - first) / 2;
                        int value = matrix[mid / n][mid % n];

                        if (value == target)
                                return true;
                        else
 
 if (value < target)
                                first = mid + 1;
                        else
                                last = mid;
                }
                return false;
        }
};