413. Arithmetic Slices ⭐

Medium
An integer array 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.
Given an integer array nums, return the number of arithmetic subarrays of nums.
A subarray is a contiguous subsequence of the array.
Example 1:
Input: nums = [1,2,3,4]
Output:
3
Explanation:
We have 3 arithmetic slices in nums: [1, 2, 3], [2, 3, 4] and [1,2,3,4] itself.
Example 2:
Input: nums = [1]
Output:
0
Constraints:
  • 1 <= nums.length <= 5000
  • -1000 <= nums[i] <= 1000

解題

使用 DP 的概念 來記錄。
func numberOfArithmeticSlices(nums []int) int {
dp := make([]int, len(nums))
res := 0
​
for i:=2; i<len(nums); i++ {
if nums[i] - nums[i-1] == nums[i-1] - nums[i-2] {
dp[i] = dp[i-1] + 1
}
res += dp[i]
}
​
return res
}
​