text: # Time:  O(n)
  # Space: O(n)
  
  import collections
  
  
  # Hash solution.
  class Solution(object):
      def isReflected(self, points):
          """
          :type points: List[List[int]]
          :rtype: bool
          """
          if not points:
              return True
          groups_by_y = collections.defaultdict(set)
          left, right = float("inf"), float("-inf")
          for p in points:
              groups_by_y[p[1]].add(p[0])
              left, right = min(left, p[0]), max(right, p[0])
          mid = left + right
          for group in groups_by_y.values():
              for x in group:
                  if mid - x not in group:
                      return False
          return True
  
  
  # Time:  O(nlogn)
  # Space: O(n)
  # Two pointers solution.
  class Solution2(object):
      def isReflected(self, points):
          """
          :type points: List[List[int]]
          :rtype: bool
          """
          if not points:
              return True
          points.sort()
          # Space: O(n)
          points[len(points)/2:] = sorted(points[len(points)/2:], \
                                          lambda x, y: y[1] - x[1] if x[0] == y[0] else \
                                                       x[0] - y[0])
          mid = points[0][0] + points[-1][0]
          left, right = 0, len(points) - 1
          while left <= right:
              if (mid != points[left][0] + points[right][0]) or \
                 (points[left][0] != points[right][0] and \
                  points[left][1] != points[right][1]):
                  return False
              left += 1
              right -= 1
          return True
  