设X[0:n-1]和Y[0:n-1]为两个数组，每个数组中都有n个已经排好序的数字，试设计一个O（logn）时间的算法，找出X和Y的2n个数的中位数。-Set X [0: n-1] and Y [0: n-1] for the two arrays, each array has n good order have been ranked the number and try to design a O (logn) time algorithm to find of the X and Y in the 2n-digit number.