LeetCode 413. Arithmetic Slices

Description

https://leetcode.com/problems/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.

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

Explanation

Iterate each number and to find all arithmetic slices starting from each number.

Python Solution

class Solution:
    def numberOfArithmeticSlices(self, nums: List[int]) -> int:
        if len(nums) < 3:
            return 0
        
        results = []
        
        for i in range(len(nums) - 1):
            prev = nums[i]
            
            diff = None
            
            subarrays = [nums[i]]
            
            
            for j in range(i + 1, len(nums)):
                if diff == None:
                    diff = nums[j] - prev
                    prev = nums[j]
                    subarrays.append(nums[j])
                else:              
                    if nums[j] - prev == diff:
                        subarrays.append(nums[j])
                        prev = nums[j]
                        
                        if len(subarrays) >= 3:
                            results.append(list(subarrays))                                            
                    else:
                        break
        
                
        return len(results)
  • Time Complexity: O(N).
  • Space Complexity: O(N).

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