LeetCode – Line Reflection (Java)

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Given n points on a 2D plane, find if there is such a line parallel to y-axis that reflects 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)?

Java Solution

For this problem, we first find the smallest and largest x-value for all points and get the line’s x-axis is (minX + maxX) / 2, then for each point, check if each point has a reflection points in the set.

public boolean isReflected(int[][] points) {
    if(points==null || points.length<2)
        return true;
 
    HashMap<Integer, HashSet<Integer>> map = new HashMap<Integer, HashSet<Integer>>();
 
    int min=Integer.MAX_VALUE;
    int max=Integer.MIN_VALUE;
 
    for(int[] arr: points){
        min = Math.min(min, arr[0]);
        max = Math.max(max, arr[0]);
        HashSet<Integer> set = map.get(arr[0]);
        if(set==null){
            set = new HashSet<Integer>();
            map.put(arr[0], set);
        }
 
        set.add(arr[1]);
 
    }
 
    int y = min+max;
 
    for(int[] arr: points){
        int left = arr[0];
        int right = y-left;
        if(map.get(right)==null || !map.get(right).contains(arr[1])){
            return false;
        }
    }
 
    return true;
}

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