README_EN.md

2501. Longest Square Streak in an Array

中文文档

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

Python3

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

class Solution:
    def longestSquareStreak(self, nums: List[int]) -> int:
        @cache
        def dfs(x):
            if x not in s:
                return 0
            return 1 + dfs(x * x)

        s = set(nums)
        ans = max(dfs(x) for x in nums)
        return -1 if ans < 2 else 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;
    }
}

class Solution {
    private Map<Integer, Integer> f = new HashMap<>();
    private Set<Integer> s = new HashSet<>();

    public int longestSquareStreak(int[] nums) {
        for (int v : nums) {
            s.add(v);
        }
        int ans = 0;
        for (int v : nums) {
            ans = Math.max(ans, dfs(v));
        }
        return ans < 2 ? -1 : ans;
    }

    private int dfs(int x) {
        if (!s.contains(x)) {
            return 0;
        }
        if (f.containsKey(x)) {
            return f.get(x);
        }
        int ans = 1 + dfs(x * x);
        f.put(x, ans);
        return ans;
    }
}

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;
    }
};

class Solution {
public:
    int longestSquareStreak(vector<int>& nums) {
        unordered_set<long long> s(nums.begin(), nums.end());
        int ans = 0;
        unordered_map<int, int> f;
        function<int(int)> dfs = [&](int x) -> int {
            if (!s.count(x)) return 0;
            if (f.count(x)) return f[x];
            long long t = 1ll * x * x;
            if (t > INT_MAX) return 1;
            f[x] = 1 + dfs(x * x);
            return f[x];
        };
        for (int& v : nums) ans = max(ans, dfs(v));
        return ans < 2 ? -1 : ans;
    }
};

Go

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
}

func longestSquareStreak(nums []int) (ans int) {
	s := map[int]bool{}
	for _, v := range nums {
		s[v] = true
	}
	f := map[int]int{}
	var dfs func(int) int
	dfs = func(x int) int {
		if !s[x] {
			return 0
		}
		if v, ok := f[x]; ok {
			return v
		}
		f[x] = 1 + dfs(x*x)
		return f[x]
	}
	for _, v := range nums {
		if t := dfs(v); ans < t {
			ans = t
		}
	}
	if ans < 2 {
		return -1
	}
	return ans
}

...