**Longest Substring with At Most Two Distinct Characters**\nGiven a string s, return the length of the longest substring that contains at most two distinct characters. This problem will test your understanding of the sliding window technique.\n\n**Function Signature**: `def length_of_longest_substring_two_distinct(s: str) -> int:`\n\n**Example 1**: \n**Input:** s = "eceba" \n**Output:** 3 \n**Explanation:** The longest substring is "ece" with length 3.\n\n**Example 2**: \n**Input:** s = "ccaabbb" \n**Output:** 5 \n**Explanation:** The longest substring is "aabbb" with length 5.\n\n**Constraints:** \n- 1 <= s.length <= 10^5 \n- s consists of English letters.

**LeetCode #159 - Longest Substring with At Most Two Distinct Characters**\nGiven a string s, return the length of the longest substring that contains at most two distinct characters. This problem requires an understanding of the sliding window technique to efficiently find the maximum length of the substring under the specified constraint.\n**Function Signature:** `def length_of_longest_substring_two_distinct(s: str) -> int:`\n**Example 1:**\nInput: `s = "eceba"`\nOutput: `3`\nExplanation: The substring is "ece" which contains two distinct characters.\n**Example 2:**\nInput: `s = "aa"`\nOutput: `2`\nExplanation: The substring is "aa" which contains one distinct character.\n**Constraints:**\n- `1 <= s.length <= 10^5`\n- `s` consists of English letters, digits, symbols, and spaces.

**Design a URL Shortener Service**\nURL shorteners are widely used for concisely hiding long URLs to make links easily shareable. Your task is to design a URL shortener service like bit.ly. Consider the requirements of URL storage, generating short keys, managing redirection, handling collisions, and providing analytics. Describe the major components, databases, and technologies you would use to implement this service at scale.\n**Functionality Requirements:** 1. Shorten a URL 2. Redirect to the original URL 3. Track statistics (clicks, etc.)\n**Non-Functional Requirements:** 1. High availability 2. Low latency 3. Secure access\n**Considerations:** Scalability and performance in handling high volume traffic.