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, 97, 7, 7, 73, -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.
思路：
依次取差，用來尋找差值的位置。對每組等差數列，計算子數列個數。
題解：
int numberOfArithmeticSlices(const std::vector<int>& A) {    int lastDelta = std::numeric_limits<int>::max();    int currentLongestSliceLength(2);    int totalSlices(0);    auto numSubSlices = [](int sliceLength) {        // For a slice, e.g. 1, 2, 3, 4, 5, the possible sub slices rae        //        //  1, 2, 3; 2, 3, 4; 3, 4, 5       -- (n - 3 + 1) slices        //  1, 2, 3, 4; 2, 3, 4, 5          -- (n - 4 + 1) slices        //  1, 2, 3, 4, 5                   -- (n - n + 1) slices        //        // So the number of subslices are sum(n - 3 + 1, ... n - n + 1) total        // n - 2 items, that is,        //  sum(n + 1, n + 1, ...) - sum(3, ... n),        // or        //  (n + 1) * (n - 2) - (n + 3) * (n - 2) / 2        //        // NOTE: extract (n - 2) out is unwise        if (sliceLength <= 2) {            return 0;        }        return (sliceLength + 1) * (sliceLength - 2) - (sliceLength + 3) * (sliceLength - 2) / 2;    };    for(size_t i = 1; i < A.size(); ++i) {        int delta = A[i] - A[i - 1];        if (delta == lastDelta) {            currentLongestSliceLength++;        } else {            totalSlices += numSubSlices(currentLongestSliceLength);            currentLongestSliceLength = 2;  // two elements at first        }        lastDelta = delta;    }    if (currentLongestSliceLength > 2) {        totalSlices += numSubSlices(currentLongestSliceLength);    }    return totalSlices;}