413. Arithmetic Slices 【M】

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本文介绍了一种用于寻找数组中所有等差序列切片数量的算法。等差序列需包含至少三个连续元素且每两个相邻元素之间的差值相同。文章提供了一个Python实现示例，并解释了如何正确识别符合要求的等差序列。

A sequence of number 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, these are arithmetic sequence:

1, 3, 5, 7, 9
7, 7, 7, 7
3, -1, -5, -9

The following sequence is not arithmetic.

1, 1, 2, 5, 7

A zero-indexed array A consisting of N numbers is given. A slice of that array is any pair of integers (P, Q) such that 0 <= P < Q < N.

A slice (P, Q) of array A is called arithmetic if the sequence:

A[P], A[p + 1], ..., A[Q - 1], A[Q] is arithmetic. In particular, this means that P + 1 < Q.

The function should return the number of arithmetic slices in the array A.

Example:

A = [1, 2, 3, 4]

return: 3, for 3 arithmetic slices in A: [1, 2, 3], [2, 3, 4] and [1, 2, 3, 4] itself.

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刚开始一直没想出怎么做，但是它的通过率竟然这么干

今天又看了一眼，才发现，原来自己看错题了，人家要求的序列，必须是两两相邻的，中间不能隔着，那岂不是简单多了

之前以为是隔着也可以呢

那这就很简单了

class Solution(object):
    def numberOfArithmeticSlices(self, A):
        if len(A) < 3:
            return 0
            
        res = 0
        
        i = 2
        l = 1
        while i < len(A):
            
            if (A[i-1] - A[i-2]) == (A[i] - A[i-1]):
                res += l
                l += 1
            else:
                l = 1
            i += 1
            
        return res