* the basic approach is to find Kth element of two sorted array,
 * first search the k/2 of the two array respectively, then you two
 * if am is greater than bm, than the median must be  the  k-k/2 th element of  a[start,length] b[start+k/2]
 * otherwise, the median must be the k-k/2 th element of a[start+k/2,length] b[start]