A sequence of number is called arithmetic if it consists of at least three elements and if the difference between any two consecutive elements is the same.
input = [1, 2, 3, 4， 6，8，10]
return: 3, for 3 arithmetic slices in A: [1, 2, 3], [2, 3, 4] ，[1, 2, 3, 4]，[6,8,10].
求等比数列的个数！
用两个变量， 一个记录当前的连续等比有几个， 不如123 cur =1， 234 cur =2，
再用另一个变量存储总共的可能性 1234 count = 1+2= 3;
