15.3 Insert Interval Time Limit Exceeded

[ByzanTine]

Hi, I gotta to run your solution on github.
However, I got a TLE when submit to online judge.
The corresponding testcase is something like
[[1,2],[3,4],[5,6],[7,8],[9,10],[11,12],[13,14],[15,16],[17,18] ... [19999,20000]], [0,20001]]
It cause multiple erase which make your algorithm in worst case O(n^2).
A O(n) time, O(n) space solution that use extra memory to save the result can basically pass the oj.

[ByzanTine]

Just found out dupe with #51, sorry about that.