3284. Sum of Consecutive Subarrays

We call an array arr of length n consecutive if one of the following holds:

• arr[i] - arr[i - 1] == 1 for all 1 <= i < n.
• arr[i] - arr[i - 1] == -1 for all 1 <= i < n.

The value of an array is the sum of its elements.

For example, [3, 4, 5] is a consecutive array of value 12 and [9, 8] is another of value 17. While [3, 4, 3] and [8, 6] are not consecutive.

Given an array of integers nums, return the sum of the values of all consecutive subarrays.

Since the answer may be very large, return it modulo 109 + 7.

Note that an array of length 1 is also considered consecutive.

class Solution {
public:
    int getSum(vector<int>& nums) {
        long long n = nums.size(), mod = 1e9 + 7, res = 0;
        auto calc = [&](int l, int r) {
            long long pre = 0, cnt = 0;
            for(int i = l; i <= r; i++) {
                ++cnt;
                pre = (pre + cnt * nums[i] % mod) % mod;
                res = (res + pre) % mod;
            }
        };
        int i = 0;
        while(i < n) {
            int j = i + 1;
            if(j == n or abs(nums[i] - nums[j]) != 1) {
                res = (res + nums[i]) % mod;
                i++;
            } else {
                int diff = nums[i] - nums[j];
                while(j < n and nums[j-1] - nums[j] == diff) j++;
                calc(i,j-1);
                if(j < n and abs(nums[j-1] - nums[j]) == 1) {
                    res -= nums[j-1];
                    i = j - 1;
                } else i = j;
            }
        }
        return res;
    }
};