# AC Java with explain from hint, not using average

• Thanks for StefanPochmann's test case. I found that average solution does not work for odd points. As the example StefanPochmann provided [[0,0],[1,0],[3,0]]. There is no reflection line. With average code, it returns true.

So after read the hint and StefanPochmann's solution, I made it with Java.

Just sort the points with x. Then "reflect" it by assume there is a x-axis formed by min_x and max_x. Then resorted the array. All the points should be same as the first sort.

``````public class Solution {
public boolean isReflected(int[][] points) {
if (points == null) {
return false;
}

Arrays.sort(points, new ArrComparator());

int[][] reflectedPoints = new int[points.length][2];
for (int i = 0; i < reflectedPoints.length; i++) {
reflectedPoints[i][0] = points[0][0] + points[reflectedPoints.length-1][0] - points[i][0];
reflectedPoints[i][1] = points[i][1];
}

Arrays.sort(reflectedPoints, new ArrComparator());

for (int i = 0; i < reflectedPoints.length; i++) {
if (reflectedPoints[i][0] != points[i][0] ||
reflectedPoints[i][1] != points[i][1]) {
return false;
}
}
return true;
}

public class ArrComparator implements Comparator<int[]>{
@Override
public int compare(int[] a, int b[]) {
if (a[0] == b[0]) {
return Integer.compare(a[1], b[1]);
}
return Integer.compare(a[0], b[0]);
}
}
``````

}

## ------------------------------FOLLOWING SOLUTION DOES NOT WORK-----------------

The approach is pretty simple the line has been restricted to be parallel to y axis. So we can just simply find the average of each y coordinate, which is the potential reflection x-axis.

Then just check whether all the reflection x-axis are same or not.

``````public class Solution {
public boolean isReflected(int[][] points) {
if (points == null) {
return false;
}

Map<Integer, List<Double>> aggregated = new HashMap<>();
for (int[] point : points) {
// First number for how many number are counted, second is the x average.
if (!aggregated.containsKey(point[1])) {
aggregated.put(point[1], new ArrayList<>(2));
} else {
double prevCount = aggregated.get(point[1]).get(0);
aggregated.get(point[1]).set(0, prevCount + 1);
aggregated.get(point[1]).set(1, (aggregated.get(point[1]).get(1) * prevCount + point[0]) / ++prevCount);
}
}

// Check potential x - axis are same or not.
Double prev = null;
for (List<Double> data : aggregated.values()) {
if (prev != null && !data.get(1).equals(prev)) {
return false;
}
prev = data.get(1);
}
return true;
}
``````

}

• Trivial to break, fails for example `[[0,0],[1,0],[3,0]]`.

• Thanks! I think this solution does not work......

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