题目
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
思路
动态规划，如果当前数值大于0，那么dp[i] = dp[i-1]，如果当前十位数大于10且小于等于26，那么dp[i] += dp[i-2]。
代码
python版本：
class Solution:
def numDecodings(self, s: str) -> int:
if not s or s[0] == '0':
return 0
dp = [0]*(len(s)+1)
dp[0:2] = 1, 1
for i in range(2, len(s)+1):
if int(s[i-1]) > 0:
dp[i] = dp[i-1]
if 26 >= int(s[i-2:i]) >= 10:
dp[i] += dp[i-2]
return dp[-1]
