题目：

题解：

class Solution {
public:int numberOfArithmeticSlices(vector<int> &nums) {int ans = 0;int n = nums.size();vector<unordered_map<long long, int>> f(n);for (int i = 0; i < n; ++i) {for (int j = 0; j < i; ++j) {long long d = 1LL * nums[i] - nums[j];auto it = f[j].find(d);int cnt = it == f[j].end() ? 0 : it->second;ans += cnt;f[i][d] += cnt + 1;}}return ans;}
};