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.

class Solution {
public:
    int longestSquareStreak(vector<int>& A) {
        sort(begin(A), end(A));
        unordered_set<int> us(begin(A), end(A));
        int res = 0;
        for(int i = 0; i < A.size(); i++) {
            if(!us.count(A[i])) continue;
            long long now = A[i], cnt = 0;
            while(us.count(now)) {
                cnt += 1;
                us.erase(now);
                now = now * now;
            }
            res = max(res, (int)cnt);
        }
        return res == 1 ? -1 : res;
    }
};