Technical Interview Preparation for Developers

Nigerian tech companies (Andela, Flutterwave, Paystack, Interswitch) use rigorous technical interviews. Master the fundamentals to succeed.

Interview Process at Nigerian Tech Companies

Stage 1: Recruiter Screen (20-30 min)

Tell me about yourself
Why this company?
Salary expectations
High-level technical questions

Stage 2: Technical Phone Screen (45-60 min)

Live coding on CoderPad or HackerRank
1-2 algorithm problems (Easy-Medium difficulty)
Explain your thought process out loud

Stage 3: Onsite/Virtual Technical Interview (3-5 rounds)

Coding Round 1 & 2: Data structures & algorithms
System Design: Design scalable systems (for mid/senior roles)
Behavioral Round: Past projects, teamwork, conflict resolution
Domain Round: Frontend, backend, or mobile-specific questions

Core Topics to Master

1. Data Structures

Arrays & Strings: Two pointers, sliding window, string manipulation
Linked Lists: Reversal, cycle detection, merge lists
Stacks & Queues: Implementation, parentheses validation, monotonic stack
Hash Tables: Two sum, anagram grouping, frequency counting
Trees: Binary trees, BST, DFS/BFS traversal
Graphs: DFS/BFS, shortest path, connected components
Heaps: Min/max heap, top K elements, merge K sorted lists

2. Algorithms

Sorting: Merge sort, quick sort, counting sort
Searching: Binary search and variations
Recursion & Backtracking: Permutations, combinations, N-queens
Dynamic Programming: Fibonacci, knapsack, longest subsequence
Greedy: Activity selection, minimum coins
Two Pointers: For sorted arrays and strings
Sliding Window: Maximum sum subarray, longest substring

3. Time & Space Complexity

Always analyze your solution's Big O complexity:

O(1) - Constant
O(log n) - Logarithmic (binary search)
O(n) - Linear (single loop)
O(n log n) - Merge sort, efficient sorting
O(n²) - Quadratic (nested loops)
O(2ⁿ) - Exponential (recursive fibonacci without memoization)

Common Interview Questions

Question 1: Two Sum (Easy)

Problem: Given an array of integers and a target, return indices of two numbers that add up to target.

Example: nums = [2, 7, 11, 15], target = 9 → Output: [0, 1]

Solution (Hash Map - O(n) time, O(n) space):

def twoSum(nums, target):
seen = {}  # Store value: index
for i, num in enumerate(nums):
complement = target - num
if complement in seen:
return [seen[complement], i]
seen[num] = i
return []

Question 2: Reverse Linked List (Easy-Medium)

Problem: Reverse a singly linked list.

Example: 1 → 2 → 3 → 4 → 5 becomes 5 → 4 → 3 → 2 → 1

Solution (Iterative - O(n) time, O(1) space):

def reverseList(head):
prev = None
current = head
while current:
next_node = current.next  # Save next
current.next = prev       # Reverse pointer
prev = current            # Move prev forward
current = next_node       # Move current forward
return prev

Question 3: Valid Parentheses (Medium)

Problem: Check if string of brackets is valid: (), {}, []

Example: "()" → True, "([)]" → False

Solution (Stack - O(n) time, O(n) space):

def isValid(s):
stack = []
mapping = {')': '(', '}': '{', ']': '['}
for char in s:
if char in mapping:  # Closing bracket
if not stack or stack.pop() != mapping[char]:
return False
else:  # Opening bracket
stack.append(char)
return len(stack) == 0

System Design (For Mid/Senior Roles)

Common System Design Questions:

Design a URL shortener (like bit.ly)
Design a payment processing system for Nigeria
Design WhatsApp/messaging app
Design an e-commerce platform
Design a ride-sharing app (like Uber)

System Design Framework (RADIO):

Requirements: Clarify functional and non-functional requirements
API Design: Define endpoints and data models
Database Schema: Choose SQL vs NoSQL, design tables
Implementation: High-level architecture diagram
Optimizations: Caching, load balancing, sharding

Example: Design a URL Shortener

Requirements:

Functional: Shorten URL, redirect to original, analytics
Non-functional: Low latency, high availability, scalable (1M URLs/day)

API:

POST /shorten
Body: { "original_url": "https://example.com/very/long/path" }
Response: { "short_url": "https://short.ly/abc123" }
GET /abc123
Response: 301 Redirect to original URL

Database Schema (SQL):

Table: urls
- id: BIGINT (primary key)
- short_code: VARCHAR(10) (indexed, unique)
- original_url: TEXT
- created_at: TIMESTAMP
- clicks: INT (for analytics)

Architecture:

Load Balancer → App Servers (Node.js/Python) → Database (PostgreSQL)
Redis cache for frequently accessed URLs
Base62 encoding for short codes (a-zA-Z0-9 = 62 characters)

Preparation Strategy

8-Week Study Plan:

Weeks 1-2: Arrays, strings, hash tables (30 problems)
Weeks 3-4: Linked lists, stacks, queues, trees (30 problems)
Weeks 5-6: Graphs, recursion, backtracking, DP (30 problems)
Weeks 7-8: Mock interviews, system design, review mistakes

Resources:

LeetCode: Practice 100+ Easy/Medium problems (focus on Nigerian company tags)
NeetCode 150: Curated list of essential problems
System Design Primer (GitHub): Free comprehensive guide
Pramp/Interviewing.io: Free mock interviews with peers

Interview Day Tips

Clarify requirements: Ask questions before coding
Think out loud: Explain your approach step-by-step
Start with brute force: Then optimize
Test your code: Walk through examples, edge cases
Handle hints gracefully: Interviewers want to help—collaborate
Manage time: Don't spend 40 minutes on one problem

Behavioral Questions for Tech Roles

Prepare STAR method answers for:

"Tell me about a challenging bug you debugged"
"Describe a time you disagreed with a teammate"
"How do you handle tight deadlines?"
"Tell me about a project you're proud of"

Nigerian Tech Companies to Target

Fintech: Flutterwave, Paystack, Kuda, Piggyvest
E-commerce: Jumia, Konga
Talent/HR Tech: Andela, Jobberman
Logistics: Kobo360, Gokada
Healthcare: Helium Health, mDoc

Salary ranges: Junior (₦200k-400k), Mid (₦450k-800k), Senior (₦900k-1.5M+)

Technical Interview Preparation for Developers