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