flowchart TB
    subgraph Singly[Singly Linked List
One-way ➡️]
        S1[HEAD] --> S2["Node: 10"]
        S2 --> S3["Node: 20"]
        S3 --> S4["Node: 30"]
        S4 --> SNULL[NULL]
    end

    subgraph Doubly[Doubly Linked List
Two-way ⬅️➡️]
        DNULL1[NULL] <--> D1["Node: 10"]
        D1 <--> D2["Node: 20"]
        D2 <--> D3["Node: 30"]
        D3 <--> DNULL2[NULL]
    end

    subgraph Circular[Circular Linked List
Loop 🔄]
        C1[HEAD] --> C2["Node: 10"]
        C2 --> C3["Node: 20"]
        C3 --> C4["Node: 30"]
        C4 -.-> C2
    end

    style Singly fill:#7c3aed,color:#fff
    style Doubly fill:#3b82f6,color:#fff
    style Circular fill:#059669,color:#fff
    style S1 fill:#f59e0b,color:#fff
    style SNULL fill:#ef4444,color:#fff
    style DNULL1 fill:#ef4444,color:#fff
    style DNULL2 fill:#ef4444,color:#fff
    style C1 fill:#f59e0b,color:#fff

class Node {
  constructor(val) {
    this.val = val;     // the data
    this.next = null;   // arrow to next node
  }
}

NULL ◀── [●|10|●] ⇄ [●|20|●] ⇄ [●|30|●] ──▶ NULL

class DNode {
  constructor(val) {
    this.val = val;
    this.prev = null;   // arrow to previous
    this.next = null;   // arrow to next
  }
}

   ┌──────────────────────────────┐
   ▼                              │
[10|●] ──▶ [20|●] ──▶ [30|●] ─────┘
 HEAD