• Time:O(n^2)
• Space:O(n^2)

C++

``````class Solution {
public:
int numberOfArithmeticSlices(vector<int>& nums) {
const int n = nums.size();
int ans = 0;
// dp[i][j] := # of subseqs end w/ nums[j] nums[i]
vector<vector<int>> dp(n, vector<int>(n));
unordered_map<long, vector<int>> numToIndices;

for (int i = 0; i < n; ++i)
numToIndices[nums[i]].push_back(i);

for (int i = 0; i < n; ++i)
for (int j = 0; j < i; ++j) {
const long target = nums[j] * 2L - nums[i];
if (numToIndices.count(target))
for (const int k : numToIndices[target])
if (k < j)
dp[i][j] += (dp[j][k] + 1);
ans += dp[i][j];
}

return ans;
}
};
``````

JAVA

``````class Solution {
public int numberOfArithmeticSlices(int[] nums) {
final int n = nums.length;
int ans = 0;
// dp[i][j] := # of subseqs end w/ nums[j] nums[i]
int[][] dp = new int[n][n];
Map<Long, List<Integer>> numToIndices = new HashMap<>();

for (int i = 0; i < n; ++i) {
numToIndices.putIfAbsent((long) nums[i], new ArrayList<>());
}

for (int i = 0; i < n; ++i)
for (int j = 0; j < i; ++j) {
final long target = nums[j] * 2L - nums[i];
if (numToIndices.containsKey(target))
for (final int k : numToIndices.get(target))
if (k < j)
dp[i][j] += (dp[j][k] + 1);
ans += dp[i][j];
}

return ans;
}
}
``````