Problem Description

You are given n accounts, labeled from 0 to n-1. An oracle function, hasMessaged(a, b), is available, which returns true if account a has sent at least one message to account b, and false otherwise.

A "spammer" is defined as an account that satisfies two conditions:

1. It has sent at least one message to every other account (i.e., all accounts j where j != a).
2. It has received messages from no other account (i.e., for all accounts j where j != a, hasMessaged(j, a) is false).

Your task is to find and return the label of a spammer account if one exists. If no spammer is found, return -1.

Constraints and Deliverables

Your solution must adhere to the following complexity constraints:

• Time Complexity: O(n) calls to the hasMessaged oracle.
• Space Complexity: O(1) extra space.

You are required to:

• Provide the algorithm.
• Prove its correctness.
• Analyze its time and space complexity.
• Discuss edge cases, specifically:
  • n equals 0 or 1.
  • Scenarios involving multiple potential spammer candidates.
  • The implications of self-messages (i.e., hasMessaged(a, a)).