License Key Formatting

License Key Formatting

What is this problem about?

The "License Key Formatting interview question" involves cleaning and reformatting a string. You are given a string s (a license key with dashes) and an integer k. You need to reformat the string such that each group contains exactly k characters, except for the first group which can be shorter. All letters must be converted to uppercase, and groups must be separated by dashes. This "License Key Formatting coding problem" is a test of string manipulation and careful indexing.

Why is this asked in interviews?

Google has been known to ask this question frequently. It tests a candidate's proficiency with "String interview pattern" logic and their ability to handle formatting constraints. It evaluates whether you can efficiently build a new string while ignoring existing separators and applying new ones based on a specific count.

Algorithmic pattern used

The most efficient approach is to process the string from right to left. By going backwards, you can easily ensure that every group has exactly k characters, and whatever is left over naturally falls into the first group at the end of your processing. You maintain a counter and insert a dash every time the counter reaches k, except for the very last characters. Finally, you reverse the result.

Example explanation

String: "5F3Z-2e-9-w", k = 4

1. Remove dashes and uppercase: "5F3Z2E9W"
2. Process backwards:
   • Take 4: "2E9W". Insert dash.
   • Take 4: "5F3Z".
3. Result: "5F3Z-2E9W" If the string was "2-5g-3-J" and k=2:

• Backwards: "3J", dash, "5G", dash, "2".
• Result: "2-5G-3J".

Common mistakes candidates make

• Forward processing complexity: Trying to process from left to right requires calculating the size of the first group ahead of time, which is more error-prone.
• String concatenation overhead: Using + in a loop in languages with immutable strings, which results in O(n^2) time. Using a StringBuilder or list is preferred.
• Trailing/Leading dashes: Accidentally leaving a dash at the very beginning or end of the formatted string.

Interview preparation tip

Whenever a problem specifies that the first group can be different but subsequent groups must be a fixed size, always try processing the data from the end. It usually makes the logic much simpler.