413. Arithmetic Slices 

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class Solution {
    public int numberOfArithmeticSlices(int[] nums) {
        if (nums.length < 3) {
            return 0;
        }

        int n = nums.length;

        // 定义 dp[i] 为以 nums[i] 结尾的等差数组的个数
        int[] dp = new int[n];
        for (int i = 2; i < n; i++) {
            if (nums[i] - nums[i - 1] == nums[i - 1] - nums[i - 2]) {
                dp[i] = dp[i - 1] + 1;
            }
        }

        int count = 0;
        for (int c : dp) {
            count += c;
        }
        return count;
    }
}

References

413. Arithmetic Slices