413. Arithmetic Slices

An integer array 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, [1,3,5,7,9], [7,7,7,7], and [3,-1,-5,-9] are arithmetic sequences.

Given an integer array nums, return the number of arithmetic subarrays of nums.

A subarray is a contiguous subsequence of the array.

 class Solution {
public:
    int numberOfArithmeticSlices(vector<int>& nums) {
        if(nums.size() <= 2) return 0;
        int res = 0, cnt = 0;
        for(int i = 2; i < nums.size(); i++) {
            if(nums[i] + nums[i-2] == 2 * nums[i-1]) cnt++;
            else {
                res += cnt * (cnt + 1) / 2;
                cnt = 0;
            }
        }
        return res + cnt * (cnt + 1) / 2;
    }
};