DISTELE - Editorial

PROBLEM LINK:

Practice
Contest: Division 1
Contest: Division 2
Contest: Division 3

Authors: Lavish Gupta
Testers: Abhinav Sharma
Editorialist: Nishank Suresh

DIFFICULTY:

Simple

PREREQUISITES:

Sorting or maps/dictionaries

PROBLEM:

A non-empty subsequence of an array is said to be good if all its elements are distinct. Given an array A, count the number of good subsequences it has.

QUICK EXPLANATION:

Let F_x denote the number of times x occurs in the array. The answer is

\prod_{x} (1 + F_x) - 1

where the product is taken over all x which appear in the array.

EXPLANATION:

Let F_x denote the number of times x appears in A.

Checking every subsequence of A for whether it’s good or not is obviously going to be too slow, since there are 2^N of them in total.

Let’s look at it differently. Consider some good subsequence S. It must have distinct elements, so let’s look at them one by one.

  • Either S contains 1, or it doesn’t. If it does contain 1, there are F_1 choices for which index this 1 is chosen to be. So, in total this gives us F_1 + 1 possibilities for 1.
  • Either S contains 2, or it doesn’t. If it does, there are F_2 choices for the index of 2. Once again, this gives us F_2 + 1 possibilities.
  • Similarly, it either doesn’t contain 3; and if it does contain 3, there are F_3 choices for it. Again, F_3 + 1 possibilities for 3.
    \vdots

So in general, considering some integer x, there are F_x + 1 possibilities provided by it — either x is not present at all, or there are F_x possible indices from where to choose x.

These choices are independent for each x, which means the total number of possibilities is (F_1 + 1)\cdot (F_2 + 1) \cdot \ldots \cdot (F_M + 1) where M is the maximum possible number in A (in this case, M = 10^6).

Note that this product also allows the choice of the empty subset - where we choose not to take any element. To exclude this case, subtract 1 from the above product.

The complexity of our solution (so far) is \mathcal{O}(M) per test case. However, there can be up to 10^5 test cases, and M = 10^6 so this isn’t quite fast enough.

OPTIMIZING FURTHER

The formula given above correctly computes the answer but is too slow. To optimize it, note that we don’t need to care about elements that don’t appear in the array, because they always contribute 1 to the product.

This allows us to optimize the computation in several ways, two of which are mentioned below.

Method 1: Maps

By far the simplest way to optimize the computation is to use maps (std::map or std::unordered_map in C++, TreeMap or HashMap in Java, dict in python) to compute the frequencies of the elements, and then just iterate over these maps. This ensures that we only consider elements that occur in the array at least once, of which there can be at most N.

Building the frequency table takes \mathcal{O}(N\log N) (or expected \mathcal{O}(N), if hashmaps are used), and the computation of the product is \mathcal{O}(N) afterward. The sum of N across all tests is bounded so this is fast enough.

Method 2: Sorting

It is also possible to accomplish this without any fancy data structures, with the help of sorting.
Note that all we really want to do is compute the frequencies of elements of A.
Suppose we sort A. Then, all equal elements will be present adjacent to each other.
This allows us to apply a simple two-pointer algorithm (or binary search) to find all segments of equal elements, and once we know those lengths we simply compute the desired product to solve the problem.

Once again, the time complexity of this is \mathcal{O}(N\log N) for the sort and \mathcal{O}(N) afterward, which is more than enough.

TIME COMPLEXITY:

\mathcal{O}(N\log N) per test case.

SOLUTIONS:

Setter's Solution (C++)

#include <bits/stdc++.h>
using namespace std;
#define ll long long
 
 
/*
------------------------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;
    while(true){
        char g=getchar();
        if(g=='-'){
            assert(fi==-1);
            is_neg=true;
            continue;
        }
        if('0'<=g && g<='9'){
            x*=10;
            x+=g-'0';
            if(cnt==0){
                fi=g-'0';
            }
            cnt++;
            assert(fi!=0 || cnt==1);
            assert(fi!=0 || is_neg==false);
 
            assert(!(cnt>19 || ( cnt==19 && fi>1) ));
        } else if(g==endd){
            if(is_neg){
                x= -x;
            }
 
            if(!(l <= x && x <= r))
            {
                cerr << l << ' ' << r << ' ' << x << '\n';
                assert(1 == 0);
            }
 
            return x;
        } else {
            assert(false);
        }
    }
}
string readString(int l,int r,char endd){
    string ret="";
    int cnt=0;
    while(true){
        char g=getchar();
        assert(g!=-1);
        if(g==endd){
            break;
        }
        cnt++;
        ret+=g;
    }
    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 = 100000;
const int MAX_SUM_N = 1000000;
 
#define fast ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
 
int sum_n = 0;
int max_n = 0;
int yess = 0;
int nos = 0;
int total_ops = 0;
ll z = 1000000007;
int freq[1000005] ;
 
void solve()
{   
    
    int n = readIntLn(1 , MAX_N) ;
    max_n = max(max_n , n) ;
    sum_n += n ;

    int arr[n] ;
    ll ans = 1 ;
    vector<int> v ;
    for(int i = 0 ; i < n-1 ; i++)
    {
        arr[i] = readIntSp(1 , 1000000);
        freq[arr[i]]++ ;
        if(freq[arr[i]] == 1)
            v.push_back(arr[i]) ;
    }
    arr[n-1] = readIntLn(1 , 1000000);
    freq[arr[n-1]]++ ;
    if(freq[arr[n-1]] == 1)
        v.push_back(arr[n-1]) ;

    for(int i = 0 ; i < v.size() ; i++)
    {
        int cnt = freq[v[i]]+1;
        ans = (ans * cnt)%z ;
        freq[v[i]] = 0 ;
    }
    cout << (ans+z-1)%z << endl ;
    return ;
}
 
signed main()
{
    // fast;
    #ifndef ONLINE_JUDGE
    freopen("inputf.txt" , "r" , stdin) ;
    freopen("outputf.txt" , "w" , stdout) ;
    freopen("error.txt" , "w" , stderr) ;
    #endif
    
    int t = 1;
    
    t = readIntLn(1,MAX_T);

    for(int i=1;i<=t;i++)
    {    
        solve() ;
    }

    assert(getchar() == -1);
    assert(sum_n <= MAX_SUM_N);
 
    cerr<<"SUCCESS\n";
    cerr<<"Tests : " << t << '\n';
    cerr<<"Sum of lengths : " << sum_n << '\n';
    cerr<<"Maximum length : " << max_n << '\n';
    // cerr<<"Total operations : " << total_ops << '\n';
    // cerr<<"Answered yes : " << yess << '\n';
    // cerr<<"Answered no : " << nos << '\n';
}
Tester's Solution (C++)
#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;
    while(true){
        char g=getchar();
        if(g=='-'){
            assert(fi==-1);
            is_neg=true;
            continue;
        }
        if('0'<=g && g<='9'){
            x*=10;
            x+=g-'0';
            if(cnt==0){
                fi=g-'0';
            }
            cnt++;
            assert(fi!=0 || cnt==1);
            assert(fi!=0 || is_neg==false);
 
            assert(!(cnt>19 || ( cnt==19 && fi>1) ));
        } else if(g==endd){
            if(is_neg){
                x= -x;
            }
 
            if(!(l <= x && x <= r))
            {
                cerr << l << ' ' << r << ' ' << x << '\n';
                assert(1 == 0);
            }
 
            return x;
        } else {
            assert(false);
        }
    }
}
string readString(int l,int r,char endd){
    string ret="";
    int cnt=0;
    while(true){
        char g=getchar();
        assert(g!=-1);
        if(g==endd){
            break;
        }
        cnt++;
        ret+=g;
    }
    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 = 1e5;
const int MAX_N = 1e5;
const int MAX_SUM_LEN = 1e5;
 
#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;

const ll MX=200000;
ll fac[MX], ifac[MX];

const ll mod = 1000000007;

ll po(ll x, ll n ){ 
    ll ans=1;
     while(n>0){
        if(n&1) ans=(ans*x)%mod;
        x=(x*x)%mod;
        n/=2;
     }
     return ans;
}

void solve()
{   
  int n;
  n = readIntLn(1, 1e5);
  sum_len+=n;
  max_n = max(max_n, n);
  ll a[n];
  for(int i=0; i<n-1; i++){
    a[i] = readIntSp(1, 1e9);
  }
  a[n-1] = readIntLn(1,1e9);

  map<ll,ll> mm;
  for(int i=0; i<n; i++){
    mm[a[i]]++;
  }

  ll ans = 1;
  for(auto h:mm){
    ans*=(h.second+1);
    ans%=mod;
  }
  ans--;
  ans+=mod;
  ans%=mod;
  cout<<ans<<'\n';

}
 
signed main()
{

    #ifndef ONLINE_JUDGE
    freopen("input.txt", "r" , stdin);
    freopen("output.txt", "w" , stdout);
    #endif
    fast;
    
    int t = 1;
    
    t = readIntLn(1,1e5);
        
    for(int i=1;i<=t;i++)
    {    
       solve();
    }
    
    assert(getchar() == -1);
    assert(sum_len <= 1e6);
 
    cerr<<"SUCCESS\n";
    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 (Python)
for _ in range(int(input())):
    n = int(input())
    a = sorted(list(map(int, input().split())))
    ans = 1
    mod = int(10**9 + 7)
    i = 0
    while i < n:
        j = i
        while j < n and a[j] == a[i]:
            j += 1
        ans *= (j - i + 1)
        ans %= mod
        i = j
    ans += mod - 1
    print(ans%mod)
5 Likes

There is no need of NlogN. count number of elements which are not repeated.Let this number be R. They will give total good sequences as 2^R.
Now store the repeated elements along with their frequencies in hashmap. Iterate through these elements one by one.
Eg: There were 8 elements before. A repeated element 1 joins the subsequence. Now there will be 8 sequences with 1 and 8 sequences without 1. The 8 sequences with 1 can be multiplied/increased acc to frequency of 1. So assume 1 appears 2 times in original array. SO these 8 sequences with 1 will become 8*2=16 sequences. And we had the 8 sequences without 1 which will not be changed.
So no of sequences after adding 1= 16+8=24. Similarly we can do for every repeated element which will take a worst case time complexity of O(N)

EDIT: subtract 1 from final answer as we do not want an empty sub-sequence

5 Likes

I look into the explanation as this :

Let ‘k’ be the number of times a distinct element occurs in the array.
For every distinct element in the array, we have two choices -
(1) Either to not take it in any subsequence, i.e, kC0 = 1
(2) Take one out of all occurrences(1 out of k), i.e, kC1 = k

So, ans = (1 + k1)(1+k2)…(1+ kq) - 1

, where q = No. of distinct elements in the array.

NOTE - “-1” is done to exclude the case where we do not choose any of the elements, i.e, the case of empty subsequence.

You can indeed use hashmaps to achieve \mathcal{O}(N), which I’ve also mentioned in the editorial (under the ‘Method 1: Maps’ section).

However, I’d like to point out a small mistake — when you use hashmaps, the complexity is not \mathcal{O}(N) in the worst case, it is expected \mathcal{O}(N). The worst case is \mathcal{O}(N^2).
Collisions are a thing, and if you’re not careful you might end up with too many of those, making your ‘correct’ program too slow.

This is especially relevant in C++, where under certain circumstances the default unordered_map can be forced to exhibit this behaviour - see this blog for how, and methods to avoid it.

tl;dr be careful when using hashmaps.

1 Like

Liked the approach of maps. Just wanted to know that why so many unwanted things in setter’s and tester’s solution, can’t understand. I mean can you make it simple like we participants code. It would be very helpful.

2 Likes

The below Code shows the Map implementation which in this case is a O(nlogn) solution. Used the map to keep track the occurrence of each number in the list.

#include<bits/stdc++.h>
using namespace std;
#define endl “\n”
typedef long long int ll;
ll mod = 1e9+7;

int main(){
ios::sync_with_stdio(0); cin.tie(0);
int t=1;
cin >> t;
while(t–){
unordered_map<int,int>mp;
int n;
cin >> n;
for(int i=0;i<n;++i){
int x;
cin >> x;
mp[x]++;
}
ll ans = 1;
for(auto &i : mp)ans = (ans * (i.second+1)) % mod;
ans%=mod;
ans–;
cout << ans << endl;
}
}

1 Like

If you are using a map then why go for Nlogn?
You can make it O(number of repeated elements) which rounds up to O(N)

Learned a new thing today. Quite Useful.
Also I would sometimes love to see a problem with edge cases designed in a way that it exploits this worst case complexity of hashmaps.

I tried to do it with finding the no. of distinct element right to an index i and then finding subsequences including that index, using map it can be achieved in O(nlogn) but its getting WA. Can anyone tell me the error??

#include<bits/stdc++.h>
#define ll long long int
#define fast_io ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define maxn 1000000007
using namespace std;
int ans(vector &v ,ll n){
ll dp[n],ca=0;

map<ll ,ll >mp;
for(ll i=0;i<n;i++){
    mp[v[i]]++;
}
if(mp.size()==1){
    return mp[v[0]];
}
else {
    for (ll i=0;i<n;i++){
        ca=(ca+n-i-(mp[v[i]])+1) % maxn;
        mp[v[i]]--;
    }
    return ca;
}

}

void fxn(){
ll n;
cin>>n;
vector v(n);
for(int i=0;i<n;i++){
cin>>v[i];
}

ll calc=ans(v,n);
cout<<calc<<endl;

}
int main(){
int t;
cin>>t;
while(t–)
{
fxn();
}
}

i tried this problem by solving using dp but wrong answer
dp[i]-> number of good subsequences ends at ith index
vector dp(n, 0);
map<int, int> m;
dp[0] = 1;
m[arr[0]] = 1;
for (int i = 1; i < n; i++) {
if (m.find(arr[i]) == m.end()) {
dp[i] = (2 * 1LL * dp[i - 1] + 1LL);
dp[i] = dp[i] % mod;
} else {
dp[i] = (2 * 1LL * dp[i - 1] - i + 1LL);
dp[i] = dp[i] % mod;
}
m[arr[i]]++;
}
cout << dp[n - 1] << endl;