LeetCode 3404: Count Special Subsequences Solution

Master LeetCode problem 3404 (Count Special Subsequences), a medium 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.

3404. Count Special Subsequences

Medium

Array Hash Table Math Enumeration

Problem Explanation

Explanation

To solve this problem, we can iterate through the array and keep track of the count of special subsequences ending at each index. We can use a 2D array dp where dp[i][j] represents the count of special subsequences ending at index i with value j. We can update the count based on the conditions mentioned in the problem. Finally, we sum up the counts in the last row of the dp array to get the total count of special subsequences in the array.

Initialize a 2D array dp of size (n, 1001) where n is the length of the input array nums.
Iterate through the input array nums and update the dp array based on the conditions for a special subsequence.
Sum up the counts in the last row of the dp array to get the total count of special subsequences.

Time complexity: O(n) Space complexity: O(n)

Solution Code

class Solution {
    public int countSpecialSubsequences(int[] nums) {
        int n = nums.length;
        int[][] dp = new int[n][1001];
        
        for (int i = 0; i < n; i++) {
            dp[i][nums[i]] = 1;
            for (int j = 1; j <= 1000; j++) {
                dp[i][j] += dp[i][j-1];
            }
        }
        
        for (int i = 1; i < n; i++) {
            for (int j = nums[i]; j <= 1000; j++) {
                dp[i][j] += dp[i-1][j-1];
            }
        }
        
        int total = 0;
        for (int j = 1; j <= 1000; j++) {
            total += dp[n-1][j];
        }
        
        return total;
    }
}

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