Given a sorted integer array without duplicates, return the summary of its ranges.
For example, given [0,1,2,4,5,7], return ["0->2","4->5","7"].
[0,1,2,4,5,7]
["0->2","4->5","7"].
228. Summary Ranges
原文：http://www.cnblogs.com/neweracoding/p/5635206.html
