Find the Lexicographically Largest String From the Box I

Find the Lexicographically Largest String From the Box I

What is this problem about?

The Find the Lexicographically Largest String From the Box I interview question tasks you with finding the "greatest" substring of a specific length within a given string. Lexicographical order is essentially dictionary order. You might be asked to find the largest substring of length $k$, or the largest possible substring if you are allowed to remove some characters.

Why is this asked in interviews?

Companies like Google, Meta, and Microsoft use this to test your understanding of Two Pointers and String Manipulation. It evaluations whether you can optimize a search by skipping unnecessary comparisons. It often touches on the Monotonic Stack interview pattern or efficient sliding window techniques to maintain the "best" result seen so far.

Algorithmic pattern used

This problem usually uses Two Pointers or a Sliding Window.

1. If the task is to find the largest substring of length $k$, you simply iterate from index 0 to $n-k$ and maintain a maxStr.
2. If the task is more complex (like finding the largest subsequence), you use a Monotonic Stack to greedily keep the largest possible characters at the beginning of your result while ensuring you have enough characters left to reach the required length.

Example explanation

String: "banana", $k=2$

1. Substrings of length 2: "ba", "an", "na", "an", "na".
2. "ba" vs "an": "ba" is larger.
3. "ba" vs "na": "na" is larger.
4. "na" vs "an": "na" is larger. Result: "na".

Common mistakes candidates make

• Inefficient Comparison: Repeatedly using substring() and string comparison ($O(N imes K)$) when a two-pointer approach could potentially skip checks.
• Character vs String: Confusing the "largest character" with the "largest string" (e.g., "b" is larger than "az", but "az" comes after "b" in some lexicographical contexts... wait, no, "b" is always after "a").
• Off-by-one: Miscalculating the loop range for the starting index.

Interview preparation tip

Understand how string comparisons work at the character level. In most languages, "abc" < "abd". For "largest" problems, greedily picking the largest character for the first position is usually the right strategy.