Because I'm silly and tried to use std::swap and make some "smart" optimizations without thinking on a HackerRank problem :(

zigzagarray.cc

// print out array in zigzag format
// time: O(n log n) = O(n log n) + O(n log n) + O(n)
// space: O(3n)
std::vector<int> zigZagArray(std::vector<int> intArray) {
  std::vector<int> v(intArray);
  // sort descending order
  std::sort(intArray.rbegin(), intArray.rend());
  // sort asc
  std::sort(v.begin(), v.end());

  std::vector<int> tmp;
  int a = 0, b = 0;
  for (int i = 0; i < v.size(); ++i) {
    if (i % 2 == 0) {
      tmp.push_back(intArray[a++]);
    } else {
      tmp.push_back(v[b++]);
    }
  }

  return tmp;
}