LeetCode Search a 2D Matrix

题目：

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 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.

class Solution {
public:
    //将矩阵看成一维数组，使用二叉搜索，复杂度为O(lgm*n)
    bool searchMatrix(vector<vector<int> > &matrix, int target) {
        int m = matrix.size(), n = matrix[0].size();
        int begin = 0, end = m*n - 1;
        while(begin <= end) {
            int mid = begin + (end - begin)/2;
            if(matrix[mid/n][mid%n] == target)
                return true;
            else if (matrix[mid/n][mid%n] > target)
                end = mid-1;
            else
                begin = mid+1;
        }
        return false;
    }
};