https://leetcode.cn/problems/longest-arithmetic-subsequence/
如果序列是：3 0 3，枚举的公差是3
对于第一个数3，它的序列长度就是他自己3
对于第二个数0，它的序列长度就行它自己0
对于第三个数，它的序列长度应该是【0，3】长度为2，第一个数也是3
一个数出现的位置越靠后，则能构成的等差数列的长度就有可能会越长
class Solution:def longestArithSeqLength(self, nums: List[int]) -> int:min_t = nums[0]max_t = nums[0]# 最大公差max_cha = -1# 最小公差min_cha = 1000ll = len(nums)for i in range(1,ll):min_cha = min(nums[i]-max_t, min_cha)max_cha = max(nums[i]-min_t, max_cha)min_t = min(nums[i],min_t)max_t = max(nums[i],max_t)res = 1for d in range(min_cha, max_cha+1):f = {}for num in nums:if num-d in f:f[num] = f[num-d] + 1else:f[num] = 1res = max(f[num],res)return res
