[容易]搜索二维矩阵

题目来源：http://www.lintcode.com/zh-cn/problem/search-a-2d-matrix/

可以accept的程序如下：

class Solution {
public:
    /**
     * @param matrix, a list of lists of integers
     * @param target, an integer
     * @return a boolean, indicate whether matrix contains target
     */
    bool searchMatrix(vector<vector<int> > &matrix, int target) {
        int n = matrix.size();
        if (n == 0) {
            return false;
        }    
        int m = matrix[0].size();      
        int start = 0, end = n * m - 1;//n行m列
        while (start + 1 < end) {
            int mid = start + (end - start) / 2;
            int row = mid / m;
            int col = mid % m;
            //从0开始计数，行和列也从0开始计数，第mid个数，在mid/m行，mid%m列
            if (matrix[row][col] < target) {
                start = mid;
            } else {
                end = mid;
            }
        }       
        if (matrix[start / m][start % m] == target) {
            return true;
        }      
        if (matrix[end / m][end % m] == target) {
            return true;
        }     
        return false;
    }
};