Lexicographically Smallest Generated String

Lexicographically Smallest Generated String

What is this problem about?

The "Lexicographically Smallest Generated String interview question" is a string reconstruction challenge. You are given a target string and a pattern. You need to construct the lexicographically smallest string that, when a certain operation is applied (like a sliding window match or a specific transformation), results in the target. This "Lexicographically Smallest Generated String coding problem" requires careful placement of characters to satisfy constraints while keeping values as low as possible.

Why is this asked in interviews?

This problem tests a candidate's ability to reason about overlapping constraints and "Greedy interview pattern" implementation. Companies like TikTok use it to evaluate how you handle conflicts between different parts of a generated string and whether you can find a valid solution in a single pass or with minimal backtracking.

Algorithmic pattern used

The general pattern is Greedy placement with validation. You initialize a result string with a placeholder (like '?'). You iterate through the target string and, based on its values, fill in the corresponding characters of your result string using the pattern. If a cell is already filled and conflicts with the current pattern, the generation is impossible. Finally, fill all remaining '?' with 'a' to ensure it's lexicographically smallest.

Example explanation

Target: "101", Pattern: "aba"

1. At index 0: Target is '1' (match). Fill result[0..2] with "aba". Result: "aba???"
2. At index 1: Target is '0' (no match). We must ensure result[1..3] is NOT "aba".
3. At index 2: Target is '1' (match). Fill result[2..4] with "aba". Result: "ababa?"
4. Check for conflicts. If index 2 was already 'c', we would return empty.
5. Fill remaining '?' with 'a'.

Common mistakes candidates make

• Ignoring overlaps: Failing to check if a character being placed conflicts with a character placed in a previous step.
• Incorrect tie-breaking: Not filling the unconstrained parts of the string with 'a' (the smallest possible character).
• Complexity issues: Using O(n*m) checks when a more efficient string matching or bitmasking approach could be used for larger patterns.

Interview preparation tip

When building a string to be "lexicographically smallest," always default to the smallest character ('a') unless a constraint forces you to use something else. Process the hard constraints first, then fill in the rest greedily.