Today I have done leetcode's January LeetCoding Challenge with cpp.

January LeetCoding Challenge 7

Description

Arithmetic Slices II - Subsequence

Given an integer array nums, return the number of all the arithmetic subsequences of nums.

A sequence of numbers is called arithmetic if it consists of at least three elements and if the difference between any two consecutive elements is the same.

• For example, [1, 3, 5, 7, 9], [7, 7, 7, 7], and [3, -1, -5, -9] are arithmetic sequences.
• For example, [1, 1, 2, 5, 7] is not an arithmetic sequence.

A subsequence of an array is a sequence that can be formed by removing some elements (possibly none) of the array.

• For example, [2,5,10] is a subsequence of [1,2,1,2,4,1,5,10].

The test cases are generated so that the answer fits in 32-bit integer.

Example 1:

Input: nums = [2,4,6,8,10]
Output: 7
Explanation: All arithmetic subsequence slices are:
[2,4,6]
[4,6,8]
[6,8,10]
[2,4,6,8]
[4,6,8,10]
[2,4,6,8,10]
[2,6,10]

Example 2:

Input: nums = [7,7,7,7,7]
Output: 16
Explanation: Any subsequence of this array is arithmetic.

Constraints:

• 1  <= nums.length <= 1000
• -231 <= nums[i] <= 231 - 1

Solution

auto speedup = [](){
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  return 0;
}();
class Solution {
public:
  int numberOfArithmeticSlices(vector<int>& nums) {
    using ll = long long;
    int len = nums.size();
    int answer = 0;
    vector<unordered_map<ll, int>> dp(len);
    for(int i = 1; i < len; ++i) {
      for(int j = 0; j < i; ++j) {
        ll diff = (ll)nums[i] - nums[j];
        int result = dp[j].count(diff) ? dp[j][diff] : 0;
        dp[i][diff] += result + 1;
        answer += result;
      }
    }

    return answer;
  }
};

// Accepted
// 101/101 cases passed (512 ms)
// Your runtime beats 63.89 % of cpp submissions
// Your memory usage beats 41.39 % of cpp submissions (155.2 MB)