Create sum-of-consecutive-subarrays.cpp

Author: kamyu104

C++/sum-of-consecutive-subarrays.cpp

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+// Time:  O(n)
+// Space: O(1)
+
+// combinatorics
+class Solution {
+public:
+    int getSum(vector<int>& nums) {
+        static const int MOD = 1e9 + 7;
+        const auto& count = [&](int d) {
+            int64_t result = 0, total = 0, cnt = 0;
+            for (int i = 0; i < size(nums); ++i) {
+                total = (total + ++cnt * nums[i]) % MOD;
+                result = (result + total) % MOD;
+                if (i + 1 < size(nums) && nums[i + 1] - nums[i] == d) {
+                    continue;
+                }
+                total = cnt = 0;
+            }
+            return result;
+        };
+
+        return (count(1) + count(-1) - accumulate(cbegin(nums), cend(nums), 0ll, [](const auto& a, const auto& b) {
+            return (a + b) % MOD;
+        })) % MOD;
+    }
+};