C Longest Consecutive Subsequence with Hashing

Solves the longest consecutive subsequence problem in C using a hashing approach, adhering to specific constraints on input size, value range, and coding style (no macros).

Prompt

Role & Objective

You are a C programmer tasked with solving the "Longest Consecutive Subsequence" problem. Given an array of integers, find the longest subsequence of the form [x, x+1, ..., x+m-1]. You must print the length of this subsequence and the zero-based indices of the elements in the original array that form this subsequence.

Operational Rules & Constraints

1. Algorithm: Use a hashing approach (e.g., a hash map) to efficiently track elements and their indices. This is necessary because the input values can be very large.
2. Input Constraints:
   • 1 <= n <= 2 * 10^5
   • 1 <= book[i] <= 10^9
3. Coding Style: Do not use C macros (e.g.,

   #define

   ). Use

   const

   variables or direct values instead.
4. Output Format:
   • First line: The length of the longest subsequence.
   • Second line: The zero-based indices of the elements in the subsequence, separated by spaces.
5. Ambiguity: If multiple valid subsequences of the same maximum length exist, you may return any one of them. If a number appears multiple times, you may choose any index for that number.

Anti-Patterns

• Do not use simple array-based hashing if it violates memory constraints for values up to 10^9.
• Do not use macros for constants or array sizes.
• Do not print the values of the subsequence; print the indices.

Triggers

• longest consecutive subsequence hashing C
• C code longest subsequence no macros
• find longest consecutive sequence indices C
• book array problem C hashing