Editorial

Let $a_0,a_1,...,a_{n-1}$ be an input array. Iterate through all possible indices $k=0,1,...,n-3$. Then, for each value of $k$, find a pair of indices $(i,j)$ such that $i>k$ and $a_i+a_j=x-a_k(i<j)$. This can be achieved on a sorted array using the two-pointer technique in linear time.

Algorithm realization

Read the input data.

cin >> n >> x;
v.resize(n);
for (i = 0; i < n; i++)
  cin >> v[i];

Sort the input array.

sort(v.begin(), v.end());

Iterate through the values $k=0,1,...,n-3$.

for (k = 0; k < n - 2; k++)
{

Find a pair of indices $(i,j)$ such that $i>k$ and $v_i+v_j=x-v_k(i<j)$. Initialize the indices $i$ and $j$.

  i = k + 1; j = n - 1;
  s = x - v[k];

Use the two-pointer technique to find the desired pair $(i,j)$.

  while (i < j)
  {
    if (v[i] + v[j] == s)
    {

If the pair $(i,j)$ is found, print the desired triplet of numbers $(v_k,v_i,v_j)$.

      printf("%d %d %d\n", v[k], v[i], v[j]);
      return 0;
    }

If $v[i]+v[j]<s$, then move pointer $i$ one position to the right;

If $v[i]+v[j]>s$, then move pointer $j$ one position to the left;

    if (v[i] + v[j] < s) i++; else j--;
  }
}

If the triplet of numbers is not found, print $-1$.

cout << -1 << endl;

Python realization

Read the input data.

n, x = map(int, input().split())
v = list(map(int, input().split()))

Sort the input list.

v.sort()

Iterate through the values $k=0,1,...,n-3$.

for k in range(n - 2):

Find a pair of indices $(i,j)$ such that $i>k$ and $v_i+v_j=x-v_k(i<j)$. Initialize the indices $i$ and $j$.

  i, j = k + 1, n – 1
  s = x - v[k]

Use the two-pointer technique to find the desired pair $(i,j)$.

  while i < j:
    if v[i] + v[j] == s:

If the pair $(i,j)$ is found, print the desired triplet of numbers $(v_k,v_i,v_j)$.

      print(v[k], v[i], v[j])
      exit(0)

If $v[i]+v[j]<s$, then move pointer $i$ one position to the right;

If $v[i]+v[j]>s$, then move pointer $j$ one position to the left;

    elif v[i] + v[j] < s:
      i += 1
    else:
      j -= 1

If the triplet of numbers is not found, print $-1$.

print(-1)