Detect Circular Money Transfers in a Transaction Graph

Question

## Problem

Given a list of transactions `[sender, receiver, amount]`, detect all **groups of accounts involved in circular money transfers** (cycles in the directed graph).

## Example
```
transactions = [[A,B,100],[B,C,50],[C,A,75],[D,E,200]]
Output: [[A,B,C]]  -- cycle: A→B→C→A
```

## Constraints
- Up to 10^5 transactions
- Account IDs are strings

## What Razorpay looks for
- Graph construction from raw data
- Cycle detection using DFS with coloring (white/grey/black)
- Returning the actual cycle nodes, not just a boolean

## Extension
What if you need to detect cycles in near real-time as transactions stream in?