413. Arithmetic Slices

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Problem

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

Code

class Solution {
    public int numberOfArithmeticSlices(int[] nums) {
        if(nums.length <= 2) return 0;

        int[] dp = new int[nums.length];
        int res = 0;

        for(int i = 2; i < nums.length; i++) {
            if(nums[i] - nums[i - 1] == nums[i - 1] - nums[i - 2]) {
                dp[i] = dp[i - 1] + 1; // dp = 1 + dp;
                res += dp[i]; // res += dp
            }
        }

        return res;
    }
}