# 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) {
}
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
}