二分。情况讨论

class Solution {
public:
    int findPos(int* p,int n,int x){
        int low=0,high=n-1,mid;
        while(low<=high){
            mid=(low+high)>>1;
            if(p[mid]<=x)low=mid+1;
            else high=mid-1;
        }
        return low;
    }
	double findK(int a[], int m, int b[], int n,int k){
		int mid1,mid2,low1=0,low2=0,high1=m-1,high2=n-1,x;
        while(low1<=high1&&low2<=high2){
            mid1=(high1+low1)>>1;
            mid2=(high2+low2)>>1;
            if(a[mid1]<b[mid2]){
                if(mid1+mid2+2==k){
					low1=mid1+1;
					high2=mid2;
				}
                else if(mid1+mid2+2<k){
                    low1=mid1+1;
                }
                else high2=mid2-1;
            }
            else{
                if(mid1+mid2+2==k){
					low2=mid2+1;
					high1=mid1;
				}
                else if(mid1+mid2+2<k){ 
                    low2=mid2+1;
                }
                else high1=mid1-1;
            }
        }
        if(low1<=high1){
		//	if(low1==high1)return a[low1];
            while(low1<=high1){
                mid1=(low1+high1)>>1;
                x=findPos(b,n,a[mid1]);
                if(x+mid1+1==k)return a[mid1];
                else if(x+mid1<k)low1=mid1+1;
                else high1=mid1-1;
            }
            return low1>=m?a[m-1]:a[low1];
        }
        else {
	//		if(low2==high2)return b[low2];
            while(low2<=high2){
                mid2=(low2+high2)>>1;
                x=findPos(a,m,b[mid2]);
                if(x+mid2+1==k)return b[mid2];
                else if(x+mid2<k)low2=mid2+1;
                else high2=mid2-1;
            }
            return low2>=n?a[n-1]:b[low2];
        }
	}
    double findMedianSortedArrays(int a[], int m, int b[], int n) {
		int k=m+n;
        if(k&1){
			return findK(a,m,b,n,k/2+1);
		}
		else{
			return (findK(a,m,b,n,k/2)+findK(a,m,b,n,k/2+1))/2.0;
		}
    }
};

[LeetCode]Median of Two Sorted Arrays查找第k数（中位数）