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

sort them:

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 n;
 int main(){
     ;
     cin>>n;
     ;i<n;i++){
         cin>>len[i];
         l+=len[i];
         ;j<len[i];j++)cin>>a[i][j];
     }
     );
     /*
     If forward l-(l/2) steps,the minimum value is the ans.
     */
     int ans;
     while(rem){
         int s=ceil(1.0*rem/n);
         ;
         ;
         //At least,there is someone forward  ceil(rem/n) steps.who should do this?
         ;i<n;i++){
             if(len[i]){
                 ,len[i]-s);
                 if(a[i][j]>mi){
                     mi=a[i][j];
                     ii=i;
                 }
             }
         }
         ,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;
     ;
 }