BPSC (Ministry of Home Affairs) Senior Computer Operator Exam: 13.09.2022 (CSE)প্রশ্ন ও সমাধান

Main limitations of Depth-First Search:

• No shortest-path guarantee: DFS dives down one branch fully before backtracking, so the first path it finds to a goal is often not the shortest one (unlike BFS).
• Risk of infinite loops: in a graph with cycles, DFS can revisit the same vertices forever if visited nodes aren't tracked.
• Unbounded depth/stack overflow: on a very deep or infinite state space, DFS can recurse arbitrarily deep, exhausting the call stack even when a much shallower solution exists elsewhere.
• Not complete for infinite graphs: if one branch is infinite and lacks the goal, DFS may never terminate and never reach the sibling branch that does contain it.

Solutions:

Problem	Fix
Infinite loops from cycles	Maintain a visited set; never re-expand a visited node
Excessive/unbounded depth	Depth-Limited Search (DLS) — stop expanding beyond a fixed depth L
DLS needs the right depth limit	Iterative Deepening DFS (IDDFS) — repeat DLS with depth 1, 2, 3, … until the goal is found; combines DFS's low memory (O(depth)) with BFS-like completeness
Need guaranteed shortest path	Use BFS instead (explores level by level)

flowchart TD
    S([Start]) --> D[DFS with visited-set and depth limit]
    D --> G{Goal found within limit?}
    G -- No --> Inc[Increase depth limit] --> D
    G -- Yes --> Done([Return path])

Exam takeaway: DFS is memory-efficient (O(depth)) but not complete/optimal alone; a visited set fixes cycles, and Iterative Deepening DFS fixes the depth/completeness problem while keeping DFS's low memory footprint.

Answer:

• SMTP (Simple Mail Transfer Protocol) is a push protocol used to send email — from a client to its outgoing mail server, and between mail servers, relaying a message toward the recipient's server. It only handles outbound delivery and cannot retrieve or browse mail.
• IMAP (Internet Message Access Protocol) is a pull protocol used to retrieve and manage email stored on the mail server — letting a client read, organise into folders, flag, and search messages while the master copy stays on the server (keeping multiple devices in sync).

Aspect	SMTP	IMAP
Purpose	Sending/relaying mail	Receiving/managing mail
Direction	Client → Server → Server → recipient	Server → Client (on request)
Default port	25 (587 for authenticated submission)	143 (993 over TLS)
Mail storage	N/A — just delivers	Stays on server, synced across devices
Contrast	—	vs. POP3, which downloads mail and often deletes it from the server

sequenceDiagram
    participant U as Sender
    participant S1 as Sender's SMTP Server
    participant S2 as Recipient's Mail Server
    participant R as Recipient Client
    U->>S1: SMTP - send message
    S1->>S2: SMTP - relay message
    R->>S2: IMAP - fetch/sync mailbox
    S2-->>R: Message list and contents

Exam takeaway: SMTP pushes mail out (sending); IMAP pulls mail in while keeping it synced on the server (receiving/managing) — complementary, not competing, protocols.