https://leetcode.com/problems/perfect-rectangle/// https://discuss.leetcode.com/topic/55944/o-n-log-n-sweep-line-solutionpublic class Solution {        public class Column implements Comparable<Column> {        int xs;        int[] rect;            public Column(int xs, int[] rect) {            this.xs = xs;            this.rect = rect;        }            public int compareTo(Column that) {            if (this.xs != that.xs) {                return this.xs - that.xs;            }            return this.rect[0] - that.rect[0];        }        }    public boolean isRectangleCover(int[][] rectangles) {        PriorityQueue<Column> pq = new PriorityQueue<Column>();        int[] border = {Integer.MAX_VALUE, Integer.MIN_VALUE};        for (int[] rect : rectangles) {            Column c1 = new Column(rect[0], rect);            Column c2 = new Column(rect[2], rect);            pq.add(c1);            pq.add(c2);            if (rect[1] < border[0]) {                border[0] = rect[1];            }            if (rect[3] > border[1]) {                border[1] = rect[3];            }        }        TreeSet<int[]> tset = new TreeSet<int[]> (new Comparator<int[]>(){            public int compare(int []rect1, int[]rect2) {                if (rect1[3] <= rect2[1]) {                    return -1;                }                else if (rect1[1] >= rect2[3]) {                    return 1;                }                else {                    return 0;                }            }        });        int yRange = 0;        while (!pq.isEmpty()) {            int xs = pq.peek().xs;            while (!pq.isEmpty() && pq.peek().xs == xs) {                Column col = pq.poll();                int[] rect = col.rect;                if (xs == rect[2]) {                    tset.remove(rect);                    yRange -= rect[3] - rect[1];                }                else {                    // xs == rect[0]                    if (!tset.add(rect)) {                        // intersect                        return false;                    }                    yRange += rect[3] - rect[1];                }            }            // if pq.isEmpty(), the right line, no need to check            if (!pq.isEmpty() && yRange != border[1] - border[0]) {                return false;            }        }        return true;    }}