LongArithSubarray.cpp

#include <iostream>
#include <iomanip>
#include <string>
#include <cmath>
#include <climits>
#include <vector>
#include <set>
#include <algorithm>
using namespace std;
#define ll             long long int
#define ld             long double
#define mod            1000000007
#define endl           "\n"
#define vi             vector<ll>
#define vs             vector<string>
#define pii            pair<ll, ll>
#define ump            unordered_map
#define mp             map
#define pq_max         priority_queue<ll>
#define pq_min         priority_queue<ll,vi,greater<ll>
#define ff             first
#define ss             second
#define mid(l,r)       (l+(r-l)/2)
#define loop(i,a,b)    for(int i=(a); i <=(b);i++)
#define looprev(i,a,b) for(int i=(a); i >=(b);i--)
#define clr(val)       memset(val,0,sizeof(val))
#define what_is(x)     cerr << #x << " is " << x << endl; 
#define OJ             \
    freopen("input.txt", "r", stdin); \
    freopen("output.txt", "w", stdout);
#define FIO ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
 
 
int main() 
{
    ll n;
    cin>>n;
 
    vector <ll> dp(n);
    loop(i,0,n-1) cin>>dp[i];

    ll pd = dp[1] - dp[0];
    ll ans = 2;
    ll j = 2;
    ll curr = 2;
    
    while(j < n){
        if (pd == dp[j] - dp[j-1]){
            curr++;
        }
        else{
            pd = dp[j] - dp[j-1];
            curr = 2;
        }
        ans = max(ans,curr);
        j++;
    }

    cout<<ans;

return 0;   
   
}