356. Line Reflection

Given n points on a 2D plane, find if there is such a line parallel to y-axis that reflect the given points.

Example 1: Given points = [[1,1],[-1,1]], return true.

Example 2: Given points = [[1,1],[-1,-1]], return false.

Follow up: Could you do better than O(n2)?

public class Solution {
    public boolean isReflected(int[][] points) {
        if(points == null || points.length <=1 ) return true;

        int left =points[0][0];
        int right = left;

        Map<Integer, Set<Integer>> map = new HashMap<>();
        for(int[] p : points){
            left = Math.min(p[0], left);
            right = Math.max(p[0], right);
            if(!map.containsKey(p[1])){
                Set<Integer> set = new HashSet<>();
                set.add(p[0]);
                map.put(p[1], set);
            }else{
                map.get(p[1]).add(p[0]);
            }
        }
        for(int[] p : points){
            Set<Integer> s = map.get(p[1]);
            if(!s.contains(left + right - p[0])) return false;
        }
        return true;

    }
}