446. Arithmetic Slices II - Subsequence

1. Question

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 answer is guaranteed to fit in 32-bit integer.

2. Example

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.

3. Constraints

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

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/arithmetic-slices-ii-subsequence 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

class Solution {
    public int numberOfArithmeticSlices(int[] nums) {
        int ans = 0;
        int n = nums.length;
        Map<Long, Integer>[] f = new Map[n];
        for (int i = 0; i < n; ++i) {
            f[i] = new HashMap<Long, Integer>();
        }
        for (int i = 0; i < n; ++i) {
            for (int j = 0; j < i; ++j) {
                long d = 1L * nums[i] - nums[j];
                int cnt = f[j].getOrDefault(d, 0);
                ans += cnt;
                f[i].put(d, f[i].getOrDefault(d, 0) + cnt + 1);
            }
        }
        return ans;
    }
}

// 解答来源于官方 本菜鸡不会