Is Longest Substring Without Repeating Characters easy or hard?

The problem is usually classified as Medium difficulty. The challenge comes from recognizing the sliding window pattern and handling duplicate characters efficiently while maintaining a valid substring window.

Longest Substring Without Repeating Characters Python/Java solution

Most Python and Java solutions implement the sliding window with a dictionary or HashMap that stores character indices. The algorithm iterates through the string once, updates the window start when duplicates appear, and tracks the maximum window length.

How to solve Longest Substring Without Repeating Characters in O(n)?

Maintain a sliding window using two pointers and a hash map storing the last index of each character. As you iterate through the string, update the left pointer whenever a duplicate appears inside the current window. This prevents rescanning characters and keeps the algorithm linear.

What is the best approach for Longest Substring Without Repeating Characters?

The optimized sliding window with a hash map is the best approach. It tracks the last seen index of each character and moves the left pointer directly past duplicates. This processes the string in one pass with O(n) time and O(min(n, k)) space, where k is the character set size.

Is Longest Substring Without Repeating Characters asked at Google/Amazon/Meta?

Longest Substring Without Repeating Characters frequently appears in interviews at companies like Amazon, Google, Meta, and Microsoft. It tests sliding window patterns, hash table usage, and efficient string processing, which are common themes in coding interviews.

What data structure is used in Longest Substring Without Repeating Characters?

The typical implementation uses a hash table (hash map) or hash set along with two pointers. The hash structure tracks characters currently in the window or their most recent index, enabling constant-time duplicate checks.

What is the time complexity of Longest Substring Without Repeating Characters?

The optimal solution runs in O(n) time using a sliding window. Each character is processed at most once while the window expands and occasionally shifts forward. Space complexity is O(min(n, k)) due to the hash table storing last seen positions.

Description Hints 1 Solution Video 6:46

3. Longest Substring Without Repeating Characters

Medium39.9% acceptance104 companies

Given a string s, find the length of the longest substring without repeating characters.

Example 1:

Input: s = "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3.

Example 2:

Input: s = "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

Example 3:

Input: s = "pwwkew"
Output: 3
Explanation: The answer is "wke", with the length of 3.
Notice that the answer must be a substring, "pwke" is a subsequence and not a substring.

Constraints: