LeetCode-413. Arithmetic Sliceshttps://leetcode.com/problems/arithmetic-slices/

题目描述

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

Constraints:

• 1 <= nums.length <= 5000
• -1000 <= nums[i] <= 1000

解题思路

【C++】

class Solution {
public:
    int numberOfArithmeticSlices(vector<int>& nums) {
        int n = nums.size(), res = 0;
        if (n < 3) {return res;}
        vector<int> dp(n, 0);
        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;
                res += dp[i];
            }
        }
        return res;
    }
};

【Java】

class Solution {
    public int numberOfArithmeticSlices(int[] nums) {
        int n = nums.length, res = 0;
        if (n < 3) {return res;}
        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;
                res += dp[i];
            }
        }
        return res;
    }
}