動機

與446差在只看每個位置的前一個就好

Problem

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: 3Explanation: 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

Sol

class Solution:
    def numberOfArithmeticSlices(self, nums: List[int]) -> int:
        dp = defaultdict(lambda : defaultdict(int))
        ret = 0
        for i in range(1,len(nums)):
            j = i-1
            diff = nums[i]-nums[j]
            ret += dp[j][diff] # Use all comb whose size is bigger than 1 to construct this case!!
            dp[i][diff] += dp[j][diff]+1
        return ret