C :

#include<stdio.h>
long long a[100010],ans[1000];
int main()
{
    long long b,x,y,m,l,c,rec;
    int i;
    while(scanf("%lld",&b)!=EOF)
    {
        for(i=0;i<b;i++)
            scanf("%d",&a[i]);
        if(b==1||b==2)
            printf("%lld\n",b);
        else
        {
            m=2;l=0;c=0;
            for(i=0;i<b-2;i++)
            {
                x=a[i+1]-a[i];
                y=a[i+2]-a[i+1];
                if(x==y)
                {
                    m++;
                    l=x;
                }
                else
                {
                    ans[c]=m;
                    m=2;
                    c++;
                }
                ans[c]=m;
            }
            rec=ans[0];
            for(i=1;i<c;i++)
            {
                if(ans[i]>rec)
                    rec=ans[i];
            }
            printf("%lld\n",rec);
        }
    }
    return 0;
}

C++ :

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <cstdlib>
#include <algorithm>
#include <vector>
#include <map>
#include <queue>
using namespace std;
#define out(v) cerr << #v << ": " << (v) << endl
#define SZ(v) ((int)(v).size())
const int maxint = -1u>>1;
template <class T> bool get_max(T& a, const T &b) {return b > a? a = b, 1: 0;}
template <class T> bool get_min(T& a, const T &b) {return b < a? a = b, 1: 0;}

const int maxn = 100000 + 5;
int seq[maxn], n;

int main() {
    //freopen("ap.out", "w", stdout);
    while(scanf("%d", &n) == 1) {
        for(int i = 0; i < n; i++) {
            scanf("%d", seq + i);
        }
        int ans = 1, s = 0, t, d;
        while(s < n - 1) {
            t = s + 1, d = seq[t] - seq[s];
            while(t < n && seq[t] - seq[t - 1] == d) t++;
            t--;
            get_max(ans, t - s + 1);
            s = t;
        }
        printf("%d\n", ans);
    }
    return 0;
}