以数组 intervals 表示若干个区间的集合，其中单个区间为 intervals[i] = [starti, endi] 。请你合并所有重叠的区间，并返回一个不重叠的区间数组，该数组需恰好覆盖输入中的所有区间。

示例 1：

输入：intervals = [[1,3],[2,6],[8,10],[15,18]]
输出：[[1,6],[8,10],[15,18]]
解释：区间 [1,3] 和 [2,6] 重叠, 将它们合并为 [1,6].

    class Solution {
        public int[][] merge(int[][] intervals) {
            if (intervals == null || intervals.length == 1) {
                return intervals;
            }
            PriorityQueue<Node> minQueue = new PriorityQueue<>((a, b) -> a.start - b.start);
            for (int i = 0; i < intervals.length; i++) {
                minQueue.add(new Node(intervals[i][0], intervals[i][1]));
            }
            int[] flag = new int[2];
            Node curNode = minQueue.poll();
            flag[0] = curNode.start;
            flag[1] = curNode.end;
            int leftMin = flag[0];
            int rightMax = flag[1];
            PriorityQueue<Node> queue = new PriorityQueue<>((a, b) -> a.start - b.start);
 

            while (!minQueue.isEmpty()) {

                Node cur = minQueue.poll();

                if (flag[1] >= cur.start) {
                    leftMin = Math.min(leftMin, cur.start);
                    rightMax = Math.max(rightMax, cur.end);
                    flag[0] = leftMin;
                    flag[1] = rightMax;
                } else if (cur.end <= rightMax) {
                    continue;
                } else {
                    queue.add(new Node(leftMin, rightMax));
                    flag[0] = cur.start;
                    flag[1] = cur.end;
                    leftMin = flag[0];
                    rightMax = flag[1];

                }

            }
            queue.add(new Node(leftMin, rightMax));

            int[][] res = new int[queue.size()][2];
            int index = 0;
            while (!queue.isEmpty()) {
                Node cur = queue.poll();
                res[index][0] = cur.start;
                res[index][1] = cur.end;
                index++;
            }
            return res;

        }

        public class Node {
            public int start;
            public int end;

            public Node(int s, int e) {
                this.start = s;
                this.end = e;
            }
        }
    }
}