class Solution {
public:
    int numberOfArithmeticSlices(vector<int>& nums) {
        int ans = 0;


        int head = 0;
        while (head + 2 < nums.size()) {
            if (nums[head + 1] - nums[head] == nums[head + 2] - nums[head + 1]) {
                int tail = head + 3;

                while (tail < nums.size() && nums[tail] - nums[tail - 1] == nums[head + 1] - nums[head]) {
                    tail++;
                }

                int len = tail - head;

                ans +=  (len - 1) * (len - 2) / 2;

                head = tail - 1;
            }
            else
                head ++;
        }

        return ans;
    }
};