Median of Two Sorted Arrays
    There are two sorted arrays nums1 and nums2 of size m and n respectively.
    Find the median of the two sorted arrays. The overall run time complexity should be O(log (m+n)).
    You may assume nums1 and nums2 cannot be both empty.

    1. Example 1:
    2. nums1 = [1, 3]
    3. nums2 = [2]
    4. The median is 2.0
    6. Example 2:
    7. nums1 = [1, 2]
    8. nums2 = [3, 4]
    9. The median is (2 + 3)/2 = 2.5

    题意：求两个有序数组的中位数。

    方法1：二分搜索
    为了方便讨论，我们首先假设两个数组的长度分别为n，m