There are two sorted arrays A and B 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)).

用二分的方法求合并数组的中值。用找第k大的值来求，不断去掉较小的一半，解法很妙。

class Solution
{
public:
   double kthNum(int A[], int m, int B[], int n, int k){
    if(m > n) return kthNum(B,n,A,m,k);
    if(m == 0) return B[k-1];
    if(k == 1) return min(A[0],B[0]);
    int pa = min(k/2,m), pb = k - pa;
    if(A[pa-1] < B[pb-1])
        return kthNum(A+pa,m-pa,B,n,k-pa);
    else if(A[pa-1] > B[pb-1])
        return kthNum(A,n,B+pb,n-pb,k-pb);
    else 
        return A[pa-1];
   }
   double findMedianSortedArrays(int A[], int m, int B[],int n){
      if((m+n)&1)
        return kthNum(A,m,B,n,(m+n)/2+1);
      else 
        return (kthNum(A,m,B,n,(m+n)/2)+kthNum(A,m,B,n,(m+n)/2+1)) /2; 
   }
};

Median of Two Sorted Arrays