Make Good

You can practice the problem [here](https://codeforces.com/contest/1270/problem/C). ## Problem An array a[0], a[1], ... , a[n - 1] of nonnegative integer numbers is said to be good if ``` a[0] + a[1] + ... + a[n - 1] = 2 * (a[0] ^ a[1] ^ ... ^ a[n - 1]) ``` Given that an array of length n, append at most 3 elements to it to make it good. ## Solution Let S be the sum of the array, which is a[0] + a[1] + ... + a[n - 1] and X be their XOR value. Then we'll have ``` S = 2 * X ``` A simple solution here is to add X and X + S to the array. ``` S = 2 * X S + X + (X + S) = 2 * (X ^ X ^ (X + S)) // X ^ X = 0 2 * (X + S) = 2 * (X + S) ``` C++ Implementation ``` ll S = 0, X = 0; REP(i, n) { ll a; cin >> a; S += a; X ^= b; } cout << 2 << "\n"; cout << X << " " << X + S << "\n"; ```