Find K nearest Points to Point P in 2-dimensional plane

Question

Source: AMAZON INTERVIEW QUESTION

Given a point P and other N points in two dimensional space, find K points

Answer 1

Solution 1

private List nearestKPoint_1(List list, final Point center, int k) {
    List ans = new ArrayList<>();
    PriorityQueue maxHeap = new PriorityQueue<>(k + 1, new Comparator() {
        @Override
        public int compare(Point o1, Point o2) {
            return distance(center, o2) - distance(center, o1);
        }
    });
    for (Point p : list) {
        maxHeap.offer(p);
        if (maxHeap.size() > k) {
            maxHeap.poll();
        }
    }
    Iterator i = maxHeap.iterator();
    while (i.hasNext()) {
        ans.add(i.next());
    }
    return ans;
}

public int distance(Point p1, Point p2) {
    return (p1.x - p2.x) * (p1.x - p2.x) + (p1.y - p2.y) * (p1.y - p2.y);
}

static class Point {
    int x;
    int y;

    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Point point = (Point) o;

        if (x != point.x) return false;
        return y == point.y;
    }

    @Override
    public int hashCode() {
        int result = x;
        result = 31 * result + y;
        return result;
    }
}

Solution 2

private List nearestKPoint_2(List list, final Point center, int k) {
    List ans = new ArrayList<>();
    Distance[] nums = new Distance[list.size()];
    for (int i = 0; i < nums.length; i++) {
        nums[i] = new Distance(distance(center, list.get(i)), i);
    }
    quickSelect(nums, k);
    for (int i = 0; i < k; i++) {
        ans.add(list.get(nums[i].i));
    }
    return ans;
}

private void quickSelect(Distance[] nums, int k) {
    int start = 0, end = nums.length - 1;
    while (start < end) {
        int p = partition(nums, start, end);
        if (p == k) {
            return;
        } else if (p < k) {
            start = p + 1;
        } else {
            end = p - 1;
        }
    }
}
private int partition(Distance[] nums, int start, int end) {
    Distance pivot = nums[start];
    int i = start, j = end + 1;
    while (true) {
        while (i < end && nums[++i].compareTo(pivot) < 0);
        while (j > start && nums[--j].compareTo(pivot) > 0);
        if (i >= j) {
            break;
        }
        swap(nums, i, j);
    }
    swap(nums, start, j);
    return j;
}

private void swap(Distance[] nums, int i, int j) {
    Distance tmp = nums[i];
    nums[i] = nums[j];
    nums[j] = tmp;
}

class Distance implements Comparable {
    int d;
    int i;

    public Distance(int d, int i) {
        this.d = d;
        this.i = i;
    }

    @Override
    public int compareTo(Distance o) {
        return this.d - o.d;
    }
}