LISTLIST - Editorial


Contest Division 1
Contest Division 2
Contest Division 3

Setter: Nishant Shah
Tester: Lavish Gupta
Editorialist: Taranpreet Singh






You are given a positive integer N and an array A of size N. There are N lists L_1, L_2 \ldots L_N. Initially, L_i = [A_i].

You can perform the following operation any number of times as long as there are at least 2 lists:

  • Select 2 (non-empty) lists L_i and L_j (i \neq j)
  • Append L_j to L_i and remove the list L_j. Note that this means L_j cannot be chosen in any future operation.

Find the minimum number of operations required to obtain a set of lists that satisfies the following conditions:

  • The first element and last element of each list are equal.
  • The first element of all the lists is the same.

Print -1 if it is not possible to achieve this via any sequence of operations.


  • If all elements are the same, no operations are needed.
  • If all elements are different, it is not possible to satisfy the above conditions.
  • Otherwise, after applying an optimal set of operations, all but one list consist of the same element x, and in the one list, only the first and last elements are x.


First of all, let’s consider base cases.

If all elements are the same

All N lists have same the first and last elements, and the first element of each list is the same, so no operations are required.

All elements are different

Since there’s no pair of equal elements, the first and the last element of any list with at least 2 elements cannot be the same. So, to satisfy the first condition, all lists must have one element only. But that violates the second condition.

Hence, if all elements are different, and N \gt 1, it is impossible to satisfy both conditions at the same time.

General case

In the general case, there always exists a solution. Since all elements are not distinct, we can always join all elements into a single list such that the first and last elements are the same. Hence, an answer is always possible in this case. We just need to find minimum number of operations.

Observation: We can see that initially, there are N lists, and after each operation, the number of lists reduces by 1. Hence, minimizing the number of operations is the same as maximizing the number of lists.

Based on the above observation, we need to maximize the number of lists, while satisfying the above conditions.

Observation 2: After an optimal set of operations, there’s exactly one list with more than one operation.
Proof: Let’s assume after all operations, there are two lists with more than one element. Let’s assume the first and last element of both lists is x.

The lists might look something like x,a,b,c,x and x,d,e,f,g,x. Instead, we can add all non-border elements of the second list into the first list, and the border elements can form two lists of length 1, increasing the number of lists. x,a,b,c,d,e,f,g,x, x and x, reducing the number of operation by 1.

Hence, all lists except the first must have only one element. They must also consist of values same as the first and last elements of the first list. Let’s assume x appears f times in the list. Two occurrences of x are used in the first list. We can form f-2 list with length 1. The total number of lists became f-1. The number of operations required becomes N - (f-1)

Hence, we can try all x, compute its frequency, and compute the minimum value of N-(f-1) over all candidates.


The time complexity is O(N*log(N)) per test case due to sorting or frequency map operations.


Setter's Solution
#include <bits/stdc++.h>
using namespace std;

#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
using namespace __gnu_pbds; 

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

int getRand(int l, int r)
    uniform_int_distribution<int> uid(l, r);
    return uid(rng);
#define int long long 
#define pb push_back
#define S second
#define F first
#define f(i,n) for(int i=0;i<n;i++)
#define fast ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define vi vector<int>
#define pii pair<int,int>
#define all(x) x.begin(),x.end()
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update> 
#define precise(x) fixed << setprecision(x) 

const int MOD = 1e9+7;

int mod_pow(int a,int b,int M = MOD)
    if(a == 0) return 0;
    b %= (M - 1);  //M must be prime here
    int res = 1;
    while(b > 0)
        if(b&1) res=(res*a)%M;
    return res;

void solve()
   int n;
   cin >> n;
   int a[n];
   f(i,n) cin >> a[i];
   map<int,int> mp;
   f(i,n) mp[a[i]]++;
   if(mp.size() == 1) cout << 0 << '\n';
   else if(mp.size() == n) cout << -1 << '\n';
       int mx = 0;
       for(auto x : mp) mx = max(mx , x.S);
       cout << n - (mx - 1) << '\n';

signed main()
    int t = 1;
    cin >> t;
Tester's Solution
#include <bits/stdc++.h>
using namespace std;
------------------------Input Checker----------------------------------
long long readInt(long long l,long long r,char endd){
    long long x=0;
    int cnt=0;
    int fi=-1;
    bool is_neg=false;
        char g=getchar();
        if('0'<=g && g<='9'){
            assert(fi!=0 || cnt==1);
            assert(fi!=0 || is_neg==false);
            assert(!(cnt>19 || ( cnt==19 && fi>1) ));
        } else if(g==endd){
                x= -x;
            if(!(l <= x && x <= r))
                cerr << l << ' ' << r << ' ' << x << '\n';
                assert(1 == 0);
            return x;
        } else {
string readString(int l,int r,char endd){
    string ret="";
    int cnt=0;
        char g=getchar();
    assert(l<=cnt && cnt<=r);
    return ret;
long long readIntSp(long long l,long long r){
    return readInt(l,r,' ');
long long readIntLn(long long l,long long r){
    return readInt(l,r,'\n');
string readStringLn(int l,int r){
    return readString(l,r,'\n');
string readStringSp(int l,int r){
    return readString(l,r,' ');
------------------------Main code starts here----------------------------------
const int MAX_T = 100000;
const int MAX_N = 200000;
const int MAX_SUM_LEN = 200000;
#define fast ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define ff first
#define ss second
#define mp make_pair
#define ll long long
int sum_len = 0;
int max_n = 0;
int yess = 0;
int nos = 0;
int total_ops = 0;

void solve()
    int n = readIntLn(1 , MAX_N);
    sum_len += n ;
    max_n = max(max_n , n) ;

    int arr[n] ;
    for(int i = 0 ; i < n-1 ; i++)
        arr[i] = readIntSp(1 , n) ;
    arr[n-1] = readIntLn(1 , n) ;

    int maxi = 0 ;
    map<int , int> m ;
    for(int i = 0 ; i < n ; i++)
        m[arr[i]]++ ;
        maxi = max(maxi , m[arr[i]]) ;
    if(n == 1)
        cout << 0 << endl ;
        return ;
    if(maxi == 1)
        cout << -1 << endl ;
        if(maxi == n)
            cout << 0 << endl ;
            cout << n-maxi+1 << endl ;
    return ;

signed main()

    #ifndef ONLINE_JUDGE
    freopen("inputf.txt", "r" , stdin);
    freopen("outputf.txt", "w" , stdout);
    int t = 1;
    t = readIntLn(1,MAX_T);
    for(int i=1;i<=t;i++)
    assert(sum_len <= MAX_SUM_LEN) ;
    assert(getchar() == -1);

    cerr<<"Tests : " << t << '\n';
    cerr<<"Sum of lengths : " << sum_len << '\n';
    cerr<<"Maximum length : " << max_n << '\n';
    // cerr<<"Total operations : " << total_ops << '\n';
    //cerr<<"Answered yes : " << yess << '\n';
    //cerr<<"Answered no : " << nos << '\n';
Editorialist's Solution
import java.util.*;
    void pre() throws Exception{}
    void solve(int TC) throws Exception{
        int N = ni();
        int[] A = new int[N];
        for(int i = 0; i< N; i++)A[i] = ni();
        int ans = N+2;
        for(int i = 0, ii = 0; i< N; i = ii){
            while(ii< N && A[i] == A[ii])ii++;  //Finding the largest block [i, ii) containing same element A[i]
            if(ii-i > 1)
                ans = Math.min(ans, N-(ii-i-1));
            if(ii-i == N)ans = 0;
        if(ans == N+2)ans = -1;
    void hold(boolean b)throws Exception{if(!b)throw new Exception("Hold right there, Sparky!");}
    static boolean multipleTC = true;
    FastReader in;PrintWriter out;
    void run() throws Exception{
        in = new FastReader();
        out = new PrintWriter(System.out);
        //Solution Credits: Taranpreet Singh
        int T = (multipleTC)?ni():1;
        pre();for(int t = 1; t<= T; t++)solve(t);
    public static void main(String[] args) throws Exception{
        new LISTLIST().run();
    int bit(long n){return (n==0)?0:(1+bit(n&(n-1)));}
    void p(Object o){out.print(o);}
    void pn(Object o){out.println(o);}
    void pni(Object o){out.println(o);out.flush();}
    String n()throws Exception{return;}
    String nln()throws Exception{return in.nextLine();}
    int ni()throws Exception{return Integer.parseInt(;}
    long nl()throws Exception{return Long.parseLong(;}
    double nd()throws Exception{return Double.parseDouble(;}

    class FastReader{
        BufferedReader br;
        StringTokenizer st;
        public FastReader(){
            br = new BufferedReader(new InputStreamReader(;

        public FastReader(String s) throws Exception{
            br = new BufferedReader(new FileReader(s));

        String next() throws Exception{
            while (st == null || !st.hasMoreElements()){
                    st = new StringTokenizer(br.readLine());
                }catch (IOException  e){
                    throw new Exception(e.toString());
            return st.nextToken();

        String nextLine() throws Exception{
            String str = "";
                str = br.readLine();
            }catch (IOException e){
                throw new Exception(e.toString());
            return str;

Feel free to share your approach. Suggestions are welcomed as always. :slight_smile:


I haven’t read the editorial yet, but i just want to know where I am going wrong in my solution

My approach is first I check if all the elements are the same or not if yes then I print 0

if not I count the frequency of every element

Then I store the maximum frequency which is even

for example if N=8 and array is [1,1,1,1,1,1,2,3] the 1 has the highest frequency then the ans will be 3+2=5

because the following list can be made


3 because tp pair each 1 with each other and 2 because to add 2,3

and if i don’t have any even frequency then i print -1

Is my approach flawed or am I missing anything?

EDIT: I think I got my mistake. I am not considering the odd number of occurrences
for example : in the above test case which i have written [1,1,1,1,1,1,2,3] it might also happen like this [1,1,1,1,1,4,2,3] for this my code will give -1 which is not the correct answer. :slight_smile: This little observation cost me 100 marks