Interview Bite

Whether you’re interviewing for a product-based startup or a big tech company like Amazon, Google, or Infosys, a solid understanding of Data Structures and Algorithms (DSA) in Python is essential.

This guide includes the most commonly asked Python DSA Interview Questions with examples, explanations, and tips to crack coding rounds with confidence.

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🔹 1. What are the most important data structures in Python?

Python offers a variety of built-in and custom data structures:

• List → Dynamic arrays
• Tuple → Immutable sequences
• Set → Unordered unique values
• Dictionary → Key-value mapping
• Queue, Stack, Heap → via collections, heapq modules
• Linked Lists, Trees, Graphs → Custom implementations

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🔹 2. Reverse a Linked List in Python (Iterative)

pythonCopyEditclass Node:
    def __init__(self, data):
        self.data = data
        self.next = None

def reverse(head):
    prev = None
    curr = head
    while curr:
        nxt = curr.next
        curr.next = prev
        prev = curr
        curr = nxt
    return prev

📌 Asked in: TCS, Wipro, Amazon

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🔹 3. Implement a Stack using a List in Python

pythonCopyEditclass Stack:
    def __init__(self):
        self.stack = []

    def push(self, item):
        self.stack.append(item)
        
    def pop(self):
        if self.stack:
            return self.stack.pop()
        return None

✅ Concepts: LIFO, time complexity O(1) for push/pop

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🔹 4. Find the first non-repeating character in a string

pythonCopyEditfrom collections import Counter

def first_unique_char(s):
    count = Counter(s)
    for ch in s:
        if count[ch] == 1:
            return ch
    return None

✅ Asked in: Cognizant, Tech Mahindra

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🔹 5. What’s the difference between List and Tuple in Python?

Feature	List	Tuple
Mutability	Mutable	Immutable
Syntax	[1, 2, 3]	(1, 2, 3)
Performance	Slower	Faster
Use Cases	Dynamic data	Fixed data

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🔹 6. Python program to check for balanced parentheses

pythonCopyEditdef is_balanced(expr):
    stack = []
    for ch in expr:
        if ch in '({[':
            stack.append(ch)
        elif ch in ')}]':
            if not stack or {')':'(', '}':'{', ']':'['}[ch] != stack.pop():
                return False
    return not stack

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🔹 7. Implement a Queue using two stacks

pythonCopyEditclass MyQueue:
    def __init__(self):
        self.in_stack = []
        self.out_stack = []

    def enqueue(self, x):
        self.in_stack.append(x)

    def dequeue(self):
        if not self.out_stack:
            while self.in_stack:
                self.out_stack.append(self.in_stack.pop())
        return self.out_stack.pop() if self.out_stack else None

📌 Common in Google, Flipkart, and HackerRank contests

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🔹 8. What is the difference between deep copy and shallow copy?

• Shallow copy: One level deep copy. Changes to nested objects affect both.
• Deep copy: Recursively copies all nested objects.

pythonCopyEditimport copy
a = [[1,2], [3,4]]
b = copy.deepcopy(a)

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🔹 9. Write a Binary Search Algorithm in Python

pythonCopyEditdef binary_search(arr, key):
    left, right = 0, len(arr)-1
    while left <= right:
        mid = (left + right) // 2
        if arr[mid] == key:
            return mid
        elif arr[mid] < key:
            left = mid + 1
        else:
            right = mid - 1
    return -1

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🔹 10. How to represent a graph using Python?

You can use a dictionary of lists to represent a graph:

pythonCopyEditgraph = {
    'A': ['B', 'C'],
    'B': ['A', 'D'],
    'C': ['A'],
    'D': ['B']
}

DFS example:

pythonCopyEditdef dfs(graph, start, visited=None):
    if visited is None:
        visited = set()
    visited.add(start)
    print(start)
    for neighbor in graph[start]:
        if neighbor not in visited:
            dfs(graph, neighbor, visited)

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📌 Interview Tips for Python DSA

• Know built-in functions: sorted(), enumerate(), zip(), etc.
• Master collections and itertools for real-world use
• Understand time/space complexity of Python built-ins
• Use list comprehensions & lambda functions for concise code
• Practice on LeetCode, GeeksforGeeks, InterviewBit

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🔚 Final Words

With Python being one of the top languages for development and data science, mastering its data structures and algorithms will give you a big edge in interviews. These questions are frequently asked in coding rounds for both freshers and experienced developers.

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