Brace Expansion II is a "Hard" version of the previous problem. In this version, braces can be nested, like "{a,b{c,d}}e". Furthermore, you can have comma-separated lists and concatenations occurring in any order. The goal is to return a sorted list of unique strings generated by the expression.
This is a classic Google interview question. it's much more than simple backtracking; it's a "parsing" problem. It tests your ability to handle recursive structures and operator precedence. It’s similar to how an expression evaluator or a compiler front-end works. It evaluates your mastery of stacks and recursive descent.
This problem can be solved using Stack-based Parsing or Recursive Descent. The two main operations are "Concatenation" (e.g., {a,b}{c,d}) and "Union" (e.g., {a,b},{c,d}). You can treat the expression as a series of nested operations. Using two stacks (one for operators/braces and one for operands/sets of strings) allows you to process the string while respecting the rules of nesting and commas.
Input: "{a,b}{c,{d,e}}"
{d,e} resolves to {d, e}.{c,{d,e}} resolves to {c, d, e}.{a,b}{c,d,e}:
ac, ad, aebc, bd, be
Result: ["ac", "ad", "ae", "bc", "bd", "be"].The most significant challenge is correctly handling the nesting levels. Candidates often try to use simple regex or string splitting, which fails for nested braces. Another common mistake is forgetting to handle duplicate strings in the final output (using a Set is essential). Managing the order of operations between commas and concatenations is also extremely tricky.
Brush up on the "Evaluate Expression" problem (e.g., solving 3 + 2 * (4 - 1)). The logic of using stacks to handle nested parentheses and operator precedence is almost identical to the logic needed for Brace Expansion II.
| Title | Difficulty | Topics | LeetCode |
|---|---|---|---|
| Number of Atoms | Hard | Solve | |
| Word Ladder II | Hard | Solve | |
| Remove Invalid Parentheses | Hard | Solve | |
| Word Ladder | Hard | Solve | |
| Find Longest Self-Contained Substring | Hard | Solve |