# MEXSUB - Editorial

Author: Ridhiman Agrawal
Tester: Rahul Dugar
Editorialist: Aman Dwivedi

Easy

# PREREQUISITES:

Dynamic Programming, Prefix Sum

# PROBLEM:

You are given an array of N integers as A_{1},A_{2},A_{3},......A_{N}. Your task is to find the number of ways to divide the array into contiguous subarrays such that:

• Each element of the given array A belongs to exactly one of the subarrays.
• There is an integer m, such that MEX of every subarray is equal to m.

MEX of a sequence is the smallest non-negative integer which doesn’t occurs in the sequence.

# QUICK EXPLANATION:

• We can notice that MEX(m) is always unique through out the whole process and is equal to the MEX of the given array.

• Any subarray of this array is valid only and only when the MEX of this subarray equals to the MEX of the given array.

• We can use Dynamic Programming approach to find out the number of ways to divide the array into contiguous subarrays such that MEX of every subarray is m.

• DP state is represented as (x), representing the number of ways to divide an array of length x, such that MEX of every subarray is m.

• DP state is calculated in order from 1 to N, and the number of ways are calculated accordingly.

• Finally output the number of ways for the sequence of length N.

# EXPLANATION:

We are given an array of N integers as A_{1},A_{2},A_{3},......A_{N}. Our task is to find the number of ways to divide the array into contiguous subarrays such that MEX of every subarray is same i.e m.

The first observation that we can draw is that MEX is always unique throughout the whole process and is equal to the MEX of the given array.

Proof

Initially, We have an array A whose MEX is equal to m. It means that all the integers less than m are present in the array.

Assume that there exists a way of dividing an array into contiguous subarrays such that MEX of every subarray is same and is less than m, say x. It means that there doesn’t exists any subarray where x is present. Since subarrays are part of array it means that there shouldn’t exist integer x in array A.

x < m

Since MEX of array is m, it means that all integers less than m exists in array. Since x<m, it means x exists in subarray which contradicts our assumption. Hence, we say that there doesn’t exist a way of diving an array into contiguous subarrays such that MEX of every subarray is same and less than m.

There doesn’t exist a subarray whose MEX is greater than m. As the MEX of given array is m, it means that the integer m doesn’t exists in array. So the maximum MEX, that any subarray can have is m.

Now, lets move towards Dynamic Programming.

Let’s create our DP as (y), which is defined as follows:

DP (y) is defined as the number of ways to divide an array of length y into contiguous subarrays such that MEX of every subarray is same and is equal to m. Now our goal is to find the subarrays whose MEX equals m, this can we done by finding subarrays where every integer less than m is present.

We set, DP[0]=1.

Now, we will find the minimum length subarray such that this subarray contains every integer less than m. Hence the MEX of this subarray is m. Suppose we have such a subarray, say S.

A_{1}, A_{2}, ......A_{x}, A_{x+1}, A_{x+2},......A_{y}
S=[A_{x+1}, A_{x+2},.........,A_{y}]

Here, S is subarray whose MEX equals m. It means if we divide the array before from x, then the MEX is still going to remain m. Hence, we can divide the array as follows:

[A_1][A_2,A_3,......,A_y] \\ [A_1,A_2][A_3,.....,A_Y] \\ ............................... \\ [A_1,A_2,....,A_x][A_{x+1},A_{x+2},.......,A_{y}]

Since, we are always sure about the last subarray that the MEX will be equal to m. And, We have already calculated the number of ways to divide an array of length less than x such that MEX is m. So, mathematical expression for the DP will be:

DP[y]=\displaystyle\sum_{i=0}^{x} DP[i],

where, DP[i] is defined as number of ways to divide an array of length i, such that MEX is m.

We can use Prefix Sum to pre-compute the sum of all DP[i] of length less than x, when we reach to length x of the given array.

Our answer will be the number of ways to divide an array of length N, into contiguous subarray such that MEX of every subarray is m. This will be represented by DP[N].

# TIME COMPLEXITY:

O(N) per testcase.

# SOLUTIONS:

Setter
#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define pb push_back

int mo = 1000000007;
int32_t main()
{
ios::sync_with_stdio(false);
cin.tie(0);
int t;
cin >> t;
while(t--)
{
int n;
cin >> n;
int fre[n + 5] = {0};
int a[n + 5];
for(int i = 1; i <= n; i++)
cin >> a[i], fre[a[i]]++;
int mex = 0;
int i = 0;
while(fre[i])
mex++, i++;
multiset<int> s;
int last_pos[mex + 5];
for(int i = 0; i < mex; i++)
last_pos[i] = -1, s.insert(-1);
if(mex == 0)
s.insert(0);
int dp[n + 5] = {0}, pre[n + 5] = {0};
dp[0] = 1, pre[0] = 1;
for(int i = 1;i <= n; i++)
{
if(a[i] < mex)
{
s.erase(s.find(last_pos[a[i]]));
last_pos[a[i]] = i;
s.insert(last_pos[a[i]]);
}
else if(mex == 0)
{
s.erase(s.begin());
s.insert(i);
}
if(*s.begin() >= 1)
{
dp[i] = pre[*s.begin() - 1];
}
pre[i] = (pre[i - 1] + dp[i])%mo;
}
cout << dp[n] << "\n";
}
}

Tester
#include <bits/stdc++.h>
using namespace std;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/rope>
using namespace __gnu_pbds;
using namespace __gnu_cxx;
#ifndef rd
#define trace(...)
#define endl '\n'
#endif
#define pb push_back
#define fi first
#define se second
#define int long long
typedef long long ll;
typedef long double f80;
#define double long double
#define pii pair<int,int>
#define pll pair<ll,ll>
#define sz(x) ((long long)x.size())
#define fr(a,b,c) for(int a=b; a<=c; a++)
#define rep(a,b,c) for(int a=b; a<c; a++)
#define trav(a,x) for(auto &a:x)
#define all(con) con.begin(),con.end()
const ll infl=0x3f3f3f3f3f3f3f3fLL;
const int infi=0x3f3f3f3f;
//const int mod=998244353;
const int mod=1000000007;
typedef vector<int> vi;
typedef vector<ll> vl;

typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> oset;
auto clk=clock();
mt19937_64 rang(chrono::high_resolution_clock::now().time_since_epoch().count());
int rng(int lim) {
uniform_int_distribution<int> uid(0,lim-1);
return uid(rang);
}
int powm(int a, int b) {
int res=1;
while(b) {
if(b&1)
res=(res*a)%mod;
a=(a*a)%mod;
b>>=1;
}
return res;
}

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;
}
assert(l<=x&&x<=r);
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) {
}
long long readIntLn(long long l, long long r) {
}
string readStringLn(int l, int r) {
}
string readStringSp(int l, int r) {
}

int a[100005],dp[100005];
int r[100005];
int latom[100005];
void solve() {
memset(dp,0,sizeof(dp));
memset(r,0,sizeof(r));
memset(latom,0,sizeof(latom));
fr(i,1,n)
if(i!=n)
else
set<int> te(a+1,a+n+1);
int meee=0;
fr(i,0,n)
if(te.count(i)==0) {
meee=i;
break;
}
if(meee==0) {
cout<<powm(2,n-1)<<endl;
return;
}
int pp=0,upto;
fr(i,1,n) {
if(a[i]<meee) {
if(latom[a[i]]==0)
pp++;
latom[a[i]]++;
}
upto=i;
if(pp==meee)
break;
}
r[1]=upto;
fr(i,1,n) {
if(a[i]<meee&&upto<=n) {
latom[a[i]]--;
while(upto<n&&latom[a[i]]==0) {
upto++;
latom[a[upto]]++;
}
if(latom[a[i]]==0)
upto++;
}
if(upto<=n)
r[i+1]=upto;
else
break;
}
dp[r[1]]=1;
fr(i,1,n) {
dp[i]+=dp[i-1];
dp[i]%=mod;
int tooo=r[i+1];
dp[tooo]+=dp[i];
dp[tooo]%=mod;
}
cout<<dp[n]<<endl;
}

signed main() {
ios_base::sync_with_stdio(0),cin.tie(0);
srand(chrono::high_resolution_clock::now().time_since_epoch().count());
cout<<fixed<<setprecision(10);
//	int t;
//	cin>>t;
fr(i,1,t)
solve();
#ifdef rd
cerr<<endl<<endl<<endl<<"Time Elapsed: "<<((double)(clock()-clk))/CLOCKS_PER_SEC<<endl;
#endif
}

Editorialist
#include<bits/stdc++.h>
using namespace std;

const int mod=1e9+7;

void solve(){
int n; cin>>n;
vector <int> a(n+1),dp(n+1),ind(n+1),prefdp(n+1);

map <int,int> m1;
int mex=0;

for(int i=1;i<=n;i++){
cin>>a[i];
dp[i]=0;
ind[i]=0;
prefdp[i]=0;
m1[a[i]]++;
}

while(m1[mex]!=0){
mex++;
}

if(mex==0){
int ans=1;
for(int i=1;i<n;i++){
ans=(ans*2)%mod;
}
cout<<ans<<"\n";
return;
}

multiset <int> index;
for(int i=0;i<mex;i++){
index.insert(0);
}

dp[0]=prefdp[0]=1;

for(int i=1;i<=n;i++){
if(a[i]<mex){
auto itr=index.find(ind[a[i]]);
index.erase(itr);
ind[a[i]]=i;
index.insert(ind[a[i]]);
}

int prev_ind=(*index.begin());
if(prev_ind!=0){
dp[i]=prefdp[prev_ind-1];
}
prefdp[i]=(prefdp[i-1]+dp[i])%mod;
}

cout<<dp[n]<<"\n";
}

int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);

int t; cin>>t;
while(t--){
solve();
}

return 0;
}



# VIDEO EDITORIAL:

14 Likes

It was definitely not of Easy Difficulty level, took me 1 hour to think then another hour to implement. Or maybe I am not just good enough for Div.1. I think it was of medium level. My complexity was also NLogN. I did something similar to the above editorial but I used a set to find the nearest index which will tell the valid partition from the current index. Then it was the same i.e using prefix sum dp of that nearest index to store dp value of the current index.
Edit:- Now I am in div.2

11 Likes

Are you sure this was EASY?

2 Likes

Man I don’t think this is even close to easy, If it is easy then this breaks my heart

4 Likes

Once a question was solved by only 40 people in total including Div1 and Div2 , If that way easy then it is too

1 Like

Are you sure this was easy ?

You can check out my solution video for the Problem Mex Subsequence (Div1 A, Div2 C, Div3 E) here if you weren’t able to solve this one.

12 Likes

I think the time complexity is nlogn.

It actually depends on the way of implementing, if we multiset for the same, yes the time complexity will be O(N*log(MEX)) as multiset will need log(MEX) time for its operation. Well you can check the tester’s code, whose way of implementing is efficient i.e. O(N).

1 Like

Hi @cherry0697, can you help me with this submission, i am unable to find a test case where it fails.

Here is a test case to give you some direction.

1
5
2 0 0 2 0

2 Likes

Thanks for the testcase.

Easy for 7 star I think​:joy:

1 Like

This is my updated solution, still WA, help needed.

The official video or written editorial is the worst I have seen. It is not at all detailed. It can be a hint. But I don’t take it as very clear for anybody to understand.

Like what is the intuition of the dp. Or how the dp is having overlapping subproblems as told by darshan in the video editorial. Nothing comes clear.

It would have been great if a proof of the solution is also uploaded. I mean proof of the dp formula

6 Likes

Hi,
I am having trouble understanding your solution.
It would be good if you can explain it a bit

1. What is the meaning of dp[i], pre_dp[i], next_occ[i]? I suppose dp[i] means the solution for the subarray till the ith index, next_occ[i] means the next occurence of the character occuring at ith index. I am not sure what pre_dp means, can you explain?
2. What is the part with 4 while statements accomplishing? Can you elaborate? It looks like the place where something could go amiss.

pre_dp[i] means sum of all dp’s upto index i (from index 0).
the logic behind the 4 while statements is …
At first i have some range where the range have all values from 0 to MEX - 1 (as well some other elements) and then i added next value A[i] , so

1. There might be a case where starting value (A[start]) might occur once again from start - end , so we can remove this first occurence.
2. A case where the starting value is > MEX , then we can remove that from our range

Since these conditions need not to be occur in the same order (i have taken 2 times each).

I have updated it , now it is good i think

UPD : got AC code
considering 2 times doesn’t make sense, we have to consider it in a different way.

2 Likes