[Leetcode]Search a 2D Matrix

思路：用經典的二分查找，先找到所在行，然後再找到所在列

class Solution {
public:
    bool searchMatrix(vector<vector<int> > &matrix, int target) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(matrix.empty()) return false;
        int row = -1;
        {
            int l = 0, r = matrix.size() - 1;
            while(l <= r)
            {
                int mid = l + (r - l) / 2;
                if(target < matrix[mid].front())
                    r = mid - 1;
                else if(target > matrix[mid].back())
                    l = mid + 1;
                else
                {
                    row = mid;
                    break;
                }
            }
        }
        if(row == -1) return false;
        int l = 0, r = matrix[0].size() - 1;
        vector<int> &arr = matrix[row];
        while(l <= r)
        {
            int mid = l + (r - l) / 2;
            if(target > arr[mid])
                l = mid + 1;
            else if(target < arr[mid])
                r = mid - 1;
            else 
                return true;
        }
        return false;
    }
};