Cracking the Code: My Amazon SDE On-Campus Interview Experience (2019)

Table of Contents#

Introduction
The Recruitment Process Breakdown
- 2.1. Online Assessment (OA)
- 2.2. Final Interview (On-Campus)
A Deep Dive into the Final Interview Round
Crucial Element: Amazon Leadership Principles
Key Takeaways and Best Practices
Conclusion
References

The Recruitment Process Breakdown#

The entire process for my campus was condensed into two main stages:

Online Assessment (OA)
Final Interview (a single round conducted on campus)

1. Online Assessment (OA)#

The first hurdle was an online test hosted on a platform like HackerRank or Amazon's own portal. It typically consists of three sections:

Debugging Questions (20 mins): You are given 7-10 code snippets with logical errors. Your task is to identify the bug and fix it within a limited time. The problems are usually straightforward, testing basic programming concepts like loops, conditionals, and array manipulations.
- Example: A function to calculate the sum of an array might have an off-by-one error in the loop.
- Best Practice: Practice common debugging scenarios. Read the code line by line, check edge cases (empty input, single element), and trace the logic with a sample input.
Coding Challenge (70 mins): This section contained 2 coding problems of medium difficulty on data structures and algorithms.
- Problem 1 (Easy-Medium): A problem related to strings or arrays. For my batch, a common question was "Two Sum" or a variation of it.
- Problem 2 (Medium): A problem involving a more complex data structure like a graph (e.g., BFS/DFS) or a tree.
- Best Practice:
  - Clarify: Before coding, restate the problem and ask clarifying questions. (e.g., "Can the array contain duplicates?", "What should be returned if no solution exists?").
  - Brute Force First: Always start by explaining a brute-force solution. This shows you understand the problem.
  - Optimize: Then, analyze the time and space complexity and propose a more optimal approach (using HashMaps, Two-pointers, etc.).
  - Code Neatly: Write clean, modular, and well-commented code.
  - Test: Walk through your code with a sample test case, including edge cases.
Work Style Assessment (15-20 mins): This is a personality test based on Amazon's 16 Leadership Principles. You'll be presented with scenarios and asked to choose the most and least likely actions you would take. There are no right or wrong answers, but it's crucial to be consistent and align your responses with principles like Customer Obsession, Ownership, and Bias for Action.

2. Final Interview (On-Campus)#

Candidates who performed well in the OA were shortlisted for a final interview. This was a 45-60 minute, face-to-face interview with one or two Amazon SDEs or managers. It was a comprehensive round focusing on coding, problem-solving, and behavioral questions.

A Deep Dive into the Final Interview Round#

Interview Structure#

The interview was seamlessly divided into three parts:

Introduction & Behavioral Questions (~15 minutes)
Data Structures & Algorithms Coding (~30 minutes)
Your Questions for the Interviewer (~5 minutes)

Problem 1: The Coding Challenge#

Question: "You are given a string. Write a function to find the first non-repeating character in it and return its index. If it doesn't exist, return -1."

Example:

Input: "amazon"
Output: 0 (because 'a' is the first character and doesn't repeat)
Input: "aabbcc"
Output: -1

My Approach & Solution:#

Clarification: I confirmed that the string could contain any character and that the comparison is case-sensitive (e.g., 'A' != 'a').
Brute Force: I first described a naive approach: for each character, check the entire string to see if it appears again. This would be O(n²) time complexity, which is inefficient for large strings.
Optimization: I proposed using a HashMap (or Dictionary) to store the frequency of each character. This is a classic space-time tradeoff.
- First Pass: Traverse the string and populate the HashMap where the key is the character and the value is its frequency count. Time: O(n)
- Second Pass: Traverse the string again. For each character, check its frequency in the HashMap. The first character with a frequency of 1 is the answer. Time: O(n)
- Overall Complexity: O(n) time and O(1) extra space (because the HashMap size is bounded by the alphabet size, which is a constant).
Coding: I then wrote the code on the whiteboard/editor.

def first_unique_char(s: str) -> int:
    """
    Finds the first non-repeating character in a string and returns its index.
    Returns -1 if no such character exists.
    """
    # Dictionary to store character frequency
    char_count = {}
    
    # First pass: count frequency of each character
    for char in s:
        char_count[char] = char_count.get(char, 0) + 1
    
    # Second pass: find the first character with count 1
    for index, char in enumerate(s):
        if char_count[char] == 1:
            return index
    
    return -1

Example Usage:

print(first_unique_char("amazon"))   # Output: 0
print(first_unique_char("aabbcc"))  # Output: -1

Testing: I dry-ran the code with the examples and an edge case (empty string).

Problem 2: The Data Structures Puzzle#

Question: "Given a binary tree, check if it is a mirror of itself (i.e., symmetric around its center)."

Example: A symmetric tree:

My Approach & Solution:#

Clarification: I confirmed the definition of a symmetric tree and what should be returned for an empty tree (trivially symmetric).
Recursive Insight: I explained that a tree is symmetric if the left subtree is a mirror reflection of the right subtree. This leads to a recursive helper function that takes two nodes and checks if they are mirrors.
- Two nodes are mirrors if:
  - Their values are equal.
  - The left subtree of the first node is a mirror of the right subtree of the second node.
  - The right subtree of the first node is a mirror of the left subtree of the second node.
Algorithm: The main function calls the helper function with the root's left and right children.

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right
 
def is_symmetric(root: TreeNode) -> bool:
    """
    Checks if a binary tree is symmetric.
    """
    if not root:
        return True
    return is_mirror(root.left, root.right)
 
def is_mirror(node1: TreeNode, node2: TreeNode) -> bool:
    """
    Helper function to check if two trees are mirrors of each other.
    """
    # Base cases: if both nodes are None, they are mirrors.
    if not node1 and not node2:
        return True
    # If only one of them is None, they are not mirrors.
    if not node1 or not node2:
        return False
    # Check: 1. Values are equal, 2. node1.left mirrors node2.right, 3. node1.right mirrors node2.left
    return (node1.val == node2.val and
            is_mirror(node1.left, node2.right) and
            is_mirror(node1.right, node2.left))

Example Usage: (The example tree above would be constructed and passed to the function.)

Discussion: The interviewer was satisfied and asked about the time/space complexity. I explained it as O(n) time, as we visit each node once, and O(h) space due to the recursion stack, where 'h' is the height of the tree.

Crucial Element: Amazon Leadership Principles#

Throughout the interview, especially during the behavioral part, the interviewer was evaluating my responses against Amazon's Leadership Principles (LPs).

For the coding problems, demonstrating "Ownership" (by thoroughly testing my code) and "Insist on the Highest Standards" (by optimizing the solution) was key.
Behavioral questions were directly linked to LPs. A common question was: "Tell me about a time you faced a difficult technical problem and how you solved it."
- My Response Structure (STAR Method):
  - Situation: Briefly describe the context. (e.g., "During a university project, we had to build a web scraper that was frequently failing due to dynamic content.")
  - Task: What was your specific goal? ("My task was to make the scraper robust and reliable.")
  - Action: What specific actions did you take? This is the most important part. ("I Dove Deep by analyzing the network logs. I Learned and Be Curious by researching headless browsers. I proposed and implemented a solution using Selenium, demonstrating Bias for Action.")
  - Result: What was the outcome? ("The scraper's success rate increased from 60% to 98%, and the project was completed on time.")

Key Takeaways and Best Practices#

Master the Basics: You must have a strong command of Data Structures (Arrays, Strings, Linked Lists, Trees, Graphs, HashMaps) and Algorithms (Recursion, Sorting, Searching, BFS, DFS).
Practice, Practice, Practice: Use platforms like LeetCode, HackerRank, and GeeksforGeeks. Focus on understanding patterns, not just memorizing solutions.
Communication is Key: Think out loud. Your interviewer wants to follow your thought process. A silent coder is a red flag.
Know the Leadership Principles Inside Out: Prepare 2-3 stories for each of the core LPs. Weave them into your answers naturally using the STAR method.
Test Your Code Thoroughly: Always discuss edge cases (empty input, negative numbers, single element) and walk through your code with an example.
Ask Insightful Questions: At the end, when asked if you have any questions, ask about the team's challenges, the company culture, or the tech stack. It shows genuine interest.

Conclusion#

The Amazon SDE interview is challenging but fair. It rigorously tests your technical competency and cultural fit. By focusing on a strong foundational knowledge, clear communication, and a deep understanding of the Leadership Principles, you can significantly increase your chances of success. Remember, the interview is not just about getting the right answer; it's about demonstrating how you think and solve problems.

Good luck with your preparation!

References#

Amazon Leadership Principles: https://www.amazon.jobs/en/principles
LeetCode - A platform for technical interview practice: https://leetcode.com/
GeeksforGeeks - CS concepts and interview preparation: https://www.geeksforgeeks.org/
Cracking the Coding Interview by Gayle Laakmann McDowell - A classic book for interview preparation.