27 NOV 446. Arithmetic Slices II - Subsequence

class Solution {
public:
 int numberOfArithmeticSlices(vector<int>& nums) {
  long long ans = 0, n = nums.size();
  vector<unordered_map<long, long>> dp(n);
  for (int i = 0; i < n; ++i) {
   for (int j = 0; j < i; ++j) {
    long long diff = (long long)nums[i] - (long long)nums[j];

    dp[i][diff]++;
    if (dp[j].count(diff)) {
     dp[i][diff] += dp[j][diff];
     ans += dp[j][diff];
    }
   }
  }
  return ans;
 }
};