413. Arithmetic Slices

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.

动态规划，遍历一次记录所有数字的差。
然后遍历并计算连续相同的数字数量temp。
当不相同时，则计算temp长度可以选择的组合数，将其添加到count。

class Solution {
    public int numberOfArithmeticSlices(int[] nums) {

        if(nums.length == 1) return 0;
        int[] difference = new int[nums.length-1];
        for(int i = 0; i < nums.length-1; i++){
            difference[i] = nums[i+1] - nums[i];
        }
        
        int count = 0;
        int temp = 1;
        int prev = difference[0];
        for(int j = 1; j < nums.length-1; j++){
            if(difference[j] == prev){
                temp++;
            }
            else{
                count += combinations(temp);
                temp = 1;
            }
            prev = difference[j];
        }
        count += combinations(temp);
        return count;
    }
    
    private int combinations(int n){
        return (n * (n-1)) / 2;
    }
}