The median of multi ascending array

Given 17 arrays,every array is ascending.The task is to get the median of all these numbers.

0 1 2 3 4           this is an array with 5 elements,the median is (5/2).

0 1 2 3 4 5        this is an array with 6 elements,the median is (6/2).

If we index an the elements from 0,the median is the element's count/2.

Input format

5
12
7 13 16 20 22 53 54 55 64 71 80 95 
10
7 19 21 58 67 77 85 89 90 96 
3
22 46 51 
3
31 43 87 
8
5 10 11 35 67 75 75 88

sort them:

5 7 7 10 11 
13 16 19 20 21 
22 22 31 35 43 
46 51 53 54 55 
58 64 67 67 71 
75 75 77 80 85 
87 88 89 90 95 
96 

36 elements(from a[0] to a[35]).The median is the a[18](the 19th)= 54.

The method above is O(n).

The method below is O(lgn).The keythought in the below method is alike with trisection.

#include<iostream>
#include<math.h>
using namespace std;
int a[17][1000];
int len[17];
int n;
int main(){
    int l=0;
    cin>>n;
    for(int i=0;i<n;i++){
        cin>>len[i];
        l+=len[i];
        for(int j=0;j<len[i];j++)cin>>a[i][j];
    }
    int rem=l-(l>>1);
    /*
    If forward l-(l/2) steps,the minimum value is the ans. 
    */
    int ans;
    while(rem){
        int s=ceil(1.0*rem/n);
        int mi=-10000;
        int ii=0;
        //At least,there is someone forward  ceil(rem/n) steps.who should do this?
        for(int i=0;i<n;i++){
            if(len[i]){
                int j=max(0,len[i]-s);
                if(a[i][j]>mi){
                    mi=a[i][j];
                    ii=i;
                }
            }
        }
        int j=max(0,len[ii]-s); 
        rem-=(len[ii]-j);
        len[ii]=j;
        ans=min(ans,a[ii][j]);//I made a mistake here,should use 'min' to update ans.
    }
    cout<<ans<<endl;
    return 0;
}