vector<int> wavePrint(vector<vector<int>> arr, int r, int c) {
  vector<int> ans;
  for (int col = 0; col < c; col++) {
    if (col & 1) {
      // odd index -> bottom to top
      for (int row = r - 1; row >= 0; row--) {
        cout << arr[row][col] << " ";
        ans.push_back(arr[row][col]);
      }
    }
    // 0 or even index -> top to bottom
    else {
      for (int row = 0; row < r; row++) {
        cout << arr[row][col] << " ";
        ans.push_back(arr[row][col]);
      }
    }
  }

  return ans;
}