2501. Longest Square Streak in an Array

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

C++

• 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;
      }
  };

JAVA

• 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;
      }
  }

PYTHON

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