413. Arithmetic Slices 【M】

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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剛開始一直沒想出怎麼做，但是它的通過率竟然這麼幹

今天又看了一眼，才發現，原來自己看錯題了，人家要求的序列，必須是兩兩相鄰的，中間不能隔着，那豈不是簡單多了

之前以爲是 隔着也可以呢

那這就很簡單了

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