Create number-of-arithmetic-triplets.cpp

Author: kamyu104

C++/number-of-arithmetic-triplets.cpp

@@ -0,0 +1,30 @@
+// Time:  O(n)
+// Space: O(n)
+
+// hash table
+class Solution {
+public:
+    int arithmeticTriplets(vector<int>& nums, int diff) {
+        unordered_set<int> lookup(cbegin(nums), cend(nums));
+        return accumulate(cbegin(nums), cend(nums), 0, [&](const auto& total, const auto&x) {
+            return total + lookup.count(x - diff) * lookup.count(x - 2 * diff);
+        });
+    }
+};
+
+// Time:  O(n)
+// Space: O(n)
+// dp
+class Solution2 {
+public:
+    int arithmeticTriplets(vector<int>& nums, int diff) {
+        int result = 0;
+        unordered_map<int, int> cnt1, cnt2;
+        for (const auto& x : nums) {
+            result += cnt2[x - diff];
+            cnt2[x] += cnt1[x - diff];
+            ++cnt1[x];
+        }
+        return result;
+    }
+};