413. Arithmetic Slices

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.

For example, these are arithmetic sequence:

1, 3, 5, 7, 9
7, 7, 7, 7
3, -1, -5, -9
The following sequence is not arithmetic.

1, 1, 2, 5, 7

A zero-indexed array A consisting of N numbers is given. A slice of that array is any pair of integers (P, Q) such that 0 <= P < Q < N.

A slice (P, Q) of array A is called arithmetic if the sequence:
A[P], A[p + 1], …, A[Q - 1], A[Q] is arithmetic. In particular, this means that P + 1 < Q.

The function should return the number of arithmetic slices in the array A.

Example:

A = [1, 2, 3, 4]

return: 3, for 3 arithmetic slices in A: [1, 2, 3], [2, 3, 4] and [1, 2, 3, 4] itself.

这一题是要求数组中是等差数列的连续子集，至少包含3个数字。
所求的arithmetic在原数组中必须是位置连续的，比如原数组若给定的是{1,2,4,3}，这里的{1,2,3}在原数组中并不连续，因此会返回0。而{1,3,5,6}中{1,3,5}位置则是连续的。
举个例子

class Solution {public:    int numberOfArithmeticSlices(vector<int>& A)     {        int count=0;        int add=0;        for(int i=2;i<A.size();i++)        {            if(A[i]-A[i-1]==A[i-1]-A[i-2])            {                add++;                count+=add;            }            else                add=0;        }        return count;    }};