LeetCode 413

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, 97, 7, 7, 73, -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: 3for 3 arithmetic slices in A: [1, 2, 3], [2, 3, 4] and [1, 2, 3, 4] itself.

solution:

分析题意，是寻找可以构成的递增数列数量。
考虑如下情况：

加入给定序列为123456
当遍历至3时，有一个递增数列。遍历至4时，新增两个递增数列。…遍历至6时，新增四个递增数列。

class Solution {public:    int numberOfArithmeticSlices(vector<int>& A) {        int n = A.size();        if(n < 3) return 0;        vector<int> tempNum(n, 0);        if(A[2]-A[1] == A[1]-A[0])            tempNum[2] = 1;        int res = tempNum[2];        for (int i = 3; i < n; i++) {            if(A[i]-A[i-1] == A[i-1]-A[i-2])                tempNum[i] = tempNum[i-1] + 1;            res += tempNum[i];        }        return res;    }};