Given a data stream input of non-negative integers `a1, a2, ..., an`, summarize the numbers seen so far as a list of disjoint intervals.

Implement the `SummaryRanges` class:

- `SummaryRanges()` Initializes the object with an empty stream.
- `void addNum(int val)` Adds the integer `val` to the stream.
- `int[][] getIntervals()` Returns a summary of the integers in the stream currently as a list of disjoint intervals `[starti, endi]`.

Example 1:

    Input
    ["SummaryRanges", "addNum", "getIntervals", "addNum", "getIntervals", "addNum", "getIntervals", "addNum", "getIntervals", "addNum", "getIntervals"]
    [[], [1], [], [3], [], [7], [], [2], [], [6], []]
    Output
    [null, null, [[1, 1]], null, [[1, 1], [3, 3]], null, [[1, 1], [3, 3], [7, 7]], null, [[1, 3], [7, 7]], null, [[1, 3], [6, 7]]]

    Explanation
    SummaryRanges summaryRanges = new SummaryRanges();
    summaryRanges.addNum(1);      // arr = [1]
    summaryRanges.getIntervals(); // return [[1, 1]]
    summaryRanges.addNum(3);      // arr = [1, 3]
    summaryRanges.getIntervals(); // return [[1, 1], [3, 3]]
    summaryRanges.addNum(7);      // arr = [1, 3, 7]
    summaryRanges.getIntervals(); // return [[1, 1], [3, 3], [7, 7]]
    summaryRanges.addNum(2);      // arr = [1, 2, 3, 7]
    summaryRanges.getIntervals(); // return [[1, 3], [7, 7]]
    summaryRanges.addNum(6);      // arr = [1, 2, 3, 6, 7]
    summaryRanges.getIntervals(); // return [[1, 3], [6, 7]]

Constraints:

- `0 <= val <= 104`
- At most `3 * 104` calls will be made to `addNum` and `getIntervals`.

Follow up: What if there are lots of merges and the number of disjoint intervals is small compared to the size of the data stream?