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2501. Longest Square Streak in an Array

Difficulty Topics

Description

You are given an integer array nums. A subsequence of nums is called a square streak if:

  • The length of the subsequence is at least 2, and
  • after sorting the subsequence, each element (except the first element) is the square of the previous number.

Return the length of the longest square streak in nums, or return -1 if there is no square streak.

A subsequence is an array that can be derived from another array by deleting some or no elements without changing the order of the remaining elements.

 

Example 1:

Input: nums = [4,3,6,16,8,2]
Output: 3
Explanation: Choose the subsequence [4,16,2]. After sorting it, it becomes [2,4,16].
- 4 = 2 * 2.
- 16 = 4 * 4.
Therefore, [4,16,2] is a square streak.
It can be shown that every subsequence of length 4 is not a square streak.

Example 2:

Input: nums = [2,3,5,6,7]
Output: -1
Explanation: There is no square streak in nums so return -1.

 

Constraints:

  • 2 <= nums.length <= 105
  • 2 <= nums[i] <= 105

Solution

longest-square-streak-in-an-array.py
class Solution:
    def longestSquareStreak(self, nums: List[int]) -> int:
        nums.sort(reverse = 1)
        count = {}

        for x in nums:
            count[x] = count.get(x * x, 0) + 1

        res = max(count.values())

        return res if res >= 2 else -1