This can be solved in many ways. One of the easiest ways is to use a for-loop and add $$$ a_i $$$ if $$$ i $$$ is odd (1-based indexing) and subtract $$$ a_i $$$ otherwise.

#include <bits/stdc++.h>

using namespace std;
#define int int64_t

int32_t main(){
    #ifndef ONLINE_JUDGE
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
    #endif
    int t;
    cin >> t;
    while(t--){
        int n;
        cin >> n;
        int ans = 0;
        for(int i = 1; i <= n; i++){
            int x;
            cin >> x;
            if(i & 1) ans += x;
            else ans -= x;
        }
        cout << ans << "\n";
    }
}

This can be solved using sets and simple conditional statements. There is an interesting solution with $$$ \oplus $$$ (bitwise-xor). Note that $$$ 1 \oplus 2 = 3 $$$. Similarly, $$$ 2 \oplus 3 = 1$$$. Hence, the result is just $$$ a \oplus b $$$.

#include <bits/stdc++.h>

using namespace std;
#define int int64_t

int32_t main(){
    #ifndef ONLINE_JUDGE
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
    #endif
    int a, b;
    cin >> a >> b;
    cout << (a ^ b) << "\n";
}

Think of how can we verify if some prefix $$$ i $$$ of string $$$ t $$$ is a valid message or not.

If we're at index $$$ i $$$ in string $$$t$$$, then let $$$ x $$$ be the length of current message. It is nothing but $$$ i + 1 $$$ (0-based indexing). Then we just have to check if prefix of length $$$ x $$$ is equal to suffix of length $$$ x $$$. This can be checked in $$$ O(1) $$$ using String Hashing.

#include <bits/stdc++.h>

using namespace std;
#define int int64_t

int getRandomNumber(int l, int r) {
    std::random_device rd;   // Seed generator
    std::mt19937 gen(rd());  // Standard mersenne_twister_engine seeded with rd()
    std::uniform_int_distribution<> dis(l, r);  // Distribution in range [l, r]
    return dis(gen);
}

struct Hash{
    int h1 = 0, h2 = 0;
    int m1 = 1e9+7, m2 = 1e9+9;
    int p;
    vector<int> pref1, pref2, power1, power2;

    Hash(int base): p(base) {}

    void build(string& a){
        power1.push_back(1);
        while(power1.size() < a.size()){
            power1.push_back((power1.back() * p) % m1);
        }
        power2.push_back(1);
        while(power2.size() < a.size()){
            power2.push_back((power2.back() * p) % m2);
        }
        pref1.resize(a.size() + 1, 0ll); pref2.resize(a.size() + 1, 0ll);
        int j = 0;
        for(int i = 0; i < a.size(); i++){
            h1 = (h1 * p % m1 + (a[i] - 'a' + 1)) % m1;
            pref1[i + 1] = h1;
            h2 = (h2 * p % m2 + (a[i] - 'a' + 1)) % m2;
            pref2[i + 1] = h2;
            j++;
        }
    }

    pair<int, int> getHash(int i, int j){
        int raw1 = pref1[j + 1] - ((pref1[i] * power1[j - i + 1]) % m1);
        int raw2 = pref2[j + 1] - ((pref2[i] * power2[j - i + 1]) % m2);
        return {(raw1 + m1) % m1, (raw2 + m2) % m2};
    }
};

int32_t main(){
    #ifndef ONLINE_JUDGE
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
    #endif
    string s;
    cin >> s;
    int n = s.size();
    int base = getRandomNumber(55, 1e8);
    Hash hash(base);
    hash.build(s);
    int ans = -1;
    for(int i = 0; i + 1 < n; i++){
        int len = max(n - i - 1, i + 1);
        if(len == i + 1) continue;
        pair<int, int> pref = hash.getHash(0, len - 1);
        pair<int, int> suff = hash.getHash(n - len, n - 1);
        if(pref.first == suff.first && pref.second == suff.second){
            ans = len - 1;
            break;
        }
    }
    if(ans == -1) cout << "NO" << "\n";
    else{
        cout << "YES" << "\n";
        for(int i = 0; i <= ans; i++) cout << s[i];
    }
}