LeetCode 3416: Subsequences with a Unique Middle Mode II Solution

Master LeetCode problem 3416 (Subsequences with a Unique Middle Mode II), a hard challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

3416. Subsequences with a Unique Middle Mode II

Hard

Array Hash Table Math Combinatorics

Problem Explanation

Explanation:

This problem asks us to find the number of subsequences in an array where the middle element is unique and has the highest frequency compared to other elements. We need to return the count modulo 10^9 + 7.

To solve this problem, we can follow these steps:

Calculate the frequency of each element in the array.
For each unique middle element, count the number of subarrays that can be formed.
Multiply the count of subarrays for each unique middle element and add it to the result. :

Solution Code

class Solution {
    public int countMiddleSubsequences(int[] nums) {
        int MOD = 1000000007;
        int n = nums.length;
        int[] freq = new int[1001];
        for (int num : nums) {
            freq[num]++;
        }
        
        long result = 0;
        for (int mid = 1; mid <= 1000; mid++) {
            if (freq[mid] == 0) continue;
            
            long left = 0, right = 0;
            for (int i = 1; i < mid; i++) {
                left += freq[i];
            }
            for (int i = mid + 1; i <= 1000; i++) {
                right += freq[i];
            }
            
            result = (result + (left * right * freq[mid]) % MOD) % MOD;
        }
        
        return (int) result;
    }
}

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