LeetCode-in-Java
446. Arithmetic Slices II - Subsequence
Hard
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
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
public class Solution {
public int numberOfArithmeticSlices(int[] arr) {
Map<Long, List<Integer>> indexes = new HashMap<>();
int[][] length = new int[arr.length][arr.length];
int count = 0;
for (int i = 0; i < arr.length; i++) {
for (int j = i + 1; j < arr.length; j++) {
List<Integer> ix = indexes.get(arr[i] - (arr[j] - (long) arr[i]));
if (ix == null) {
continue;
}
for (int k : ix) {
length[i][j] += length[k][i] + 1;
}
count += length[i][j];
}
indexes.computeIfAbsent((long) arr[i], k -> new ArrayList<>()).add(i);
}
return count;
}
}