LeetCode 2145: Count the Hidden Sequences Solution

Master LeetCode problem 2145 (Count the Hidden Sequences), 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.

2145. Count the Hidden Sequences

Medium

Array Prefix Sum

Problem Explanation

Explanation

To solve this problem, we need to find all possible hidden sequences that satisfy the given differences and lie within the inclusive range [lower, upper]. We can approach this problem using dynamic programming.

Initialize a dynamic programming array dp of size n+1 to store the count of possible hidden sequences ending at index i.
Initialize dp[0] = 1, as there is only one way to form a hidden sequence of length 1.
Iterate through the differences array and update dp[i+1] based on the valid sequences that can be formed from dp[i] for each difference.
Finally, count the total number of valid hidden sequences within the range [lower, upper] from dp.

Time Complexity: O(n) where n is the length of the differences array. Space Complexity: O(n)

Solution Code

class Solution {
    public int countHiddenSequences(int[] differences, int lower, int upper) {
        int n = differences.length;
        int[] dp = new int[n + 1];
        dp[0] = 1;

        for (int i = 0; i < n; i++) {
            for (int j = 0; j <= i; j++) {
                if (dp[j] > 0 && lower <= dp[j] + differences[i] && dp[j] + differences[i] <= upper) {
                    dp[i + 1] += dp[j];
                }
            }
        }

        int count = 0;
        for (int num : dp) {
            if (num > 0) {
                count++;
            }
        }

        return count;
    }
}

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