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2501. Longest Square Streak in an Array

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

Solutions

  • class Solution {
    public int longestSquareStreak(int[] nums) {
        Set<Integer> s = new HashSet<>();
        for (int v : nums) {
            s.add(v);
        }
        int ans = -1;
        for (int v : nums) {
            int t = 0;
            while (s.contains(v)) {
                v *= v;
                ++t;
            }
            if (t > 1) {
                ans = Math.max(ans, t);
            }
        }
        return ans;
    }
}

  • class Solution {
public:
    int longestSquareStreak(vector<int>& nums) {
        unordered_set<long long> s(nums.begin(), nums.end());
        int ans = -1;
        for (int& v : nums) {
            int t = 0;
            long long x = v;
            while (s.count(x)) {
                x *= x;
                ++t;
            }
            if (t > 1) ans = max(ans, t);
        }
        return ans;
    }
};

  • class Solution:
    def longestSquareStreak(self, nums: List[int]) -> int:
        s = set(nums)
        ans = -1
        for v in nums:
            t = 0
            while v in s:
                v *= v
                t += 1
            if t > 1:
                ans = max(ans, t)
        return ans


  • func longestSquareStreak(nums []int) int {
	s := map[int]bool{}
	for _, v := range nums {
		s[v] = true
	}
	ans := -1
	for _, v := range nums {
		t := 0
		for s[v] {
			v *= v
			t++
		}
		if t > 1 && t > ans {
			ans = t
		}
	}
	return ans
}

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