Lưu ý: Các code mẫu dưới đây chỉ mang tính tham khảo và có thể không AC được bài tập này

Code mẫu của happyboy99x

#include<iostream>
#include<cstring>
using namespace std;

const int MAX = 1e6, N = 2500;
int pos[(MAX << 2) + 1], f[N][N], a[N], n;

void enter() {
    cin >> n;
    for(int i = 0; i < n; ++i)
        cin >> a[i];
}

void solve() {
    memset(pos, -1, sizeof pos);
    int res = 0;
    for(int i = 0; i < n; ++i) {
        for(int j = i + 1; j < n; ++j) {
            int p = pos[a[j] - a[i] + (MAX << 1)];
            if(p != -1) f[i][j] = f[p][i] + 1;
            else f[i][j] = 2;
            res = max(res, f[i][j]);
        }
        pos[a[i] + (MAX << 1)] = i;
    }
    cout << res << "\n";
}

int main() {
    ios::sync_with_stdio(false);
    int tc; cin >> tc;
    for(int t = 0; t < tc; ++t) {
        enter();
        solve();
    }
    return 0;
}

Code mẫu của hieult

#include <set>
#include <map>
#include <list>
#include <cmath>
#include <queue>
#include <stack>
#include <cstdio>
#include <string>
#include <vector>
#include <cstdlib>
#include <cstring>
#include <sstream>
#include <iomanip>
#include <iostream>
#include <algorithm>
#include <fstream>
#include <ctime>
#include <deque>
#include <bitset>
#include <cctype>
#include <utility>
#include <cassert>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef unsigned int ui;
typedef unsigned long long ull;

#define Rep(i,n) for(int i = 0; i < (n); ++i)
#define Repd(i,n) for(int i = (n)-1; i >= 0; --i)
#define For(i,a,b) for(int i = (a); i <= (b); ++i)
#define Ford(i,a,b) for(int i = (a); i >= (b); --i)
#define Fit(i,v) For(__typeof((v).begin()) i = (v).begin(); i != (v).end(); ++i)
#define Fitd(i,v) For(__typeof((v).rbegin()) i = (v).rbegin(); i != (v).rend(); ++i)
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define sz(a) ((int)(a).size())
#define all(a) (a).begin(), (a).end()
#define ms(a,x) memset(a, x, sizeof(a))

template<class F, class T> T convert(F a, int p = -1) { stringstream ss; if (p >= 0) ss << fixed << setprecision(p); ss << a; T r; ss >> r; return r; }
template<class T> T gcd(T a, T b) { T r; while (b != 0) { r = a % b; a = b; b = r; } return a; }
template<class T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
template<class T> T sqr(T x) { return x * x; }
template<class T> T cube(T x) { return x * x * x; }
template<class T> int getbit(T s, int i) { return (s >> i) & 1; }
template<class T> T onbit(T s, int i) { return s | (T(1) << i); }
template<class T> T offbit(T s, int i) { return s & (~(T(1) << i)); }
template<class T> int cntbit(T s) { return s == 0 ? 0 : cntbit(s >> 1) + (s & 1); }
const int bfsz = 1 << 16; char bf[bfsz + 5]; int rsz = 0;int ptr = 0;
char gc() { if (rsz <= 0) { ptr = 0; rsz = (int) fread(bf, 1, bfsz, stdin); if (rsz <= 0) return EOF; } --rsz; return bf[ptr++]; }
void ga(char &c) { c = EOF; while (!isalpha(c)) c = gc(); }
int gs(char s[]) { int l = 0; char c = gc(); while (isspace(c)) c = gc(); while (c != EOF && !isspace(c)) { s[l++] = c; c = gc(); } s[l] = '\0'; return l; }
template<class T> bool gi(T &v) {
    v = 0; char c = gc(); while (c != EOF && c != '-' && !isdigit(c)) c = gc(); if (c == EOF) return false; bool neg = c == '-'; if (neg) c = gc();
    while (isdigit(c)) { v = v * 10 + c - '0'; c = gc(); } if (neg) v = -v; return true;
}

typedef pair<int, int> II;

const double PI = acos(-1.0);
const double eps = 1e-9;

const int inf = (int)1e9 + 5;
const ll linf = (ll)1e17 + 5;
int dr[4] = {-1, 0, +1, 0};
int dc[4] = {0, -1, 0, +1};
const ll mod = 76213ll;

#define maxn 2505
#define MAX 1000000

int vt[2000005], f[maxn][maxn], a[maxn];
int test, n;

int main(){
//  freopen("in.txt", "r", stdin);
//  freopen("out.txt", "w", stdout);
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    cin >> test;
    Rep(itest, test){
        ms(f, 0); ms(vt, 0);
        int res = 0, x;
        cin >> n;
        For(i, 1, n) cin >> a[i];
        Ford(i, n, 1){

            Ford(j, i - 1, 1){
                f[j][i] = 2;
                x = a[i] + a[j];
                if(x <= MAX && x >= -MAX && vt[x + MAX]){
                    f[j][i] = max(f[j][i], f[i][vt[x + MAX]] + 1);
                }
                res = max(res, f[j][i]);
            }

            vt[a[i] + MAX] = i;
        }
        cout << res << endl;
    }

//  cout << clock() << endl;
}

Code mẫu của skyvn97

#include <iostream>
#include <cstdio>
using namespace std;
const int Maxn=2555, Mn=1e6+10;
int a[Maxn];
int don[Mn<<1];
int cnt;
int F[Maxn][Maxn];
int n,Mx,t,k,i,j;
inline bool mid(const int& l,const int& m,const int& r) {return l<m&&m<r;}
inline void getmax(int& x,const int& val){if(x<val)x=val;}
main(){
    ios_base::sync_with_stdio(false);
    cin>>t;
    while(t--) {
        cin>>n;
        for(i=1; i<=n; i++) {
            cin>>a[i];
            }
        Mx=0;
        for(i=3; i<=n; i++) {
            don[a[1]+Mn]=cnt+1;
            for(j=2; j<i; j++) {
                if(mid(-Mn,a[i]-a[j],Mn))
                if(mid(0,k=don[a[i]-a[j]+Mn]-cnt,j)) {
                    if(F[k][j]<cnt) F[k][j]=cnt;
                    F[j][i]=F[k][j]+1;
                    getmax(Mx,F[j][i]-cnt);
                    }
                don[a[j]+Mn]=cnt+j;
                }
            }
        printf("%d\n",Mx+2);
        cnt+=n;
        }
    }