Coding Interview Preparation on Telegram: “🎯 CODING CHALLENGE #5

🎯 CODING CHALLENGE #5 - Longest Substring Without Repeating Characters
Difficulty: Medium | Asked at: Amazon, Meta, Bloomberg


Input:  "abcabcbb"
Output: 3   ("abc")

Input:  "bbbbb"
Output: 1   ("b")

💡 Hint: This screams sliding window. Keep expanding a window to the right, and when you hit a repeat, shrink from the left until the repeat is gone.

Solution:

python
def length_of_longest_substring(s):
    seen = {}
    left = 0
    max_len = 0

    for right, char in enumerate(s):
        if char in seen and seen[char] >= left:
            left = seen[char] + 1
        seen[char] = right
        max_len = max(max_len, right - left + 1)

    return max_len

Complexity: O(n) time - each character is visited by right once, and left only ever moves forward. O(min(n, alphabet size)) space for the hash map.

Common mistake: Resetting left to seen[char] + 1 even when the previous occurrence of char is OUTSIDE the current window (i.e., seen[char] < left). Without the seen[char] >= left check, you can accidentally move left backwards, which breaks the algorithm.

Sliding window is one of the highest-ROI patterns to master - it solves a huge chunk of "substring" and "subarray" problems. Comfortable with it, or still building intuition? 👇