How do I practice coding and algorithm questions?

Use PracHub's coding console to write, test, and debug your solutions in Python or JavaScript. View hints, test against sample inputs, and compare with official solutions.

What difficulty level is this coding question?

This is a Medium difficulty Coding & Algorithms question, commonly asked during Technical Screen rounds at Amazon.

What role is this question designed for?

This question is commonly asked for Data Scientist candidates at Amazon during technical interviews.

Solve Algorithmic Challenges in Online Coding Assessments

Company: Amazon

Role: Data Scientist

Category: Coding & Algorithms

Difficulty: Medium

Interview Round: Technical Screen

##### Scenario Online assessment requiring implementation of common algorithmic problems. ##### Question Given an array of integers and a target, return indices of the two numbers that add up to the target (assume exactly one solution). 2) Given a string, determine the length of the longest substring without repeating characters. ##### Hints Use hash maps for O(n) solutions; mind edge cases like empty input.

Quick Answer: This question evaluates proficiency in fundamental algorithmic problem-solving, specifically array manipulation, string processing, and the use of basic data structures along with performance and complexity reasoning.

Two Sum

Given an array of integers `nums` and an integer `target`, return the indices of the two numbers such that they add up to `target`. You may assume that each input has exactly one solution, and you may not use the same element twice. Return the answer as a list of the two indices `[i, j]` with `i < j`. Example 1: Input: nums = [2, 7, 11, 15], target = 9 Output: [0, 1] (because nums[0] + nums[1] == 9) Example 2: Input: nums = [3, 2, 4], target = 6 Output: [1, 2] Use a hash map to achieve an O(n) single-pass solution.

Constraints

2 <= len(nums) <= 10^4
-10^9 <= nums[i] <= 10^9
-10^9 <= target <= 10^9
Exactly one valid answer exists.

Examples

Input: ([2, 7, 11, 15], 9)

Expected Output: [0, 1]

Explanation: nums[0] + nums[1] = 2 + 7 = 9.

Input: ([3, 2, 4], 6)

Expected Output: [1, 2]

Explanation: nums[1] + nums[2] = 2 + 4 = 6; note nums[0]=3 alone cannot reuse itself.

Input: ([3, 3], 6)

Expected Output: [0, 1]

Explanation: Duplicate values; the two distinct indices 0 and 1 sum to 6.

Input: ([-1, -2, -3, -4, -5], -8)

Expected Output: [2, 4]

Explanation: Negatives: nums[2] + nums[4] = -3 + -5 = -8.

Input: ([0, 4, 3, 0], 0)

Expected Output: [0, 3]

Explanation: Two zeros at indices 0 and 3 sum to the target 0.

Hints

A brute-force O(n^2) approach checks every pair, but you can do better.
As you iterate, store each number's index in a hash map.
For each number, check whether target - number is already in the map; if so you have your pair.

Longest Substring Without Repeating Characters

Given a string `s`, find the length of the longest substring that contains no repeating characters. A substring is a contiguous sequence of characters within the string. Example 1: Input: s = "abcabcbb" Output: 3 (the answer is "abc", with length 3) Example 2: Input: s = "bbbbb" Output: 1 (the answer is "b") Example 3: Input: s = "pwwkew" Output: 3 (the answer is "wke"; note "pwke" is a subsequence, not a substring) Use a sliding window with a hash map of last-seen indices for an O(n) solution.

Constraints

0 <= len(s) <= 5 * 10^4
s consists of English letters, digits, symbols, and spaces.

Examples

Input: ("abcabcbb",)

Expected Output: 3

Explanation: The longest unique substring is "abc" with length 3.

Input: ("bbbbb",)

Expected Output: 1

Explanation: Every character repeats; the best window is a single "b".

Input: ("pwwkew",)

Expected Output: 3

Explanation: The longest unique substring is "wke" with length 3.

Input: ("",)

Expected Output: 0

Explanation: Empty string has no characters, so the answer is 0.

Input: ("abba",)

Expected Output: 2

Explanation: After the repeat 'b' then 'a', start must jump correctly; the best is "ab" or "ba" of length 2.

Input: ("dvdf",)

Expected Output: 3

Explanation: Tricky case: the answer is "vdf"; start must not move backward when 'd' repeats.

Input: (" ",)

Expected Output: 1

Explanation: A single space is one unique character, length 1.

Hints

Maintain a sliding window [start, i] that always contains only unique characters.
Track the most recent index where each character was seen in a hash map.
When you encounter a repeat that lies inside the current window, jump start to one past its previous index.

Quick Overview

This question evaluates proficiency in fundamental algorithmic problem-solving, specifically array manipulation, string processing, and the use of basic data structures along with performance and complexity reasoning.