Solution: Remove Invalid Parentheses

Explore how to solve the problem of removing invalid parentheses from a string using recursive backtracking. Learn to count unmatched parentheses, make removal decisions, and generate all valid expressions efficiently. This lesson helps deepen your understanding of combinatorial problem solving and backtracking techniques applied to coding interviews.

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Statement
Solution
- Time complexity
- Space complexity

Statement

You are given a string, s, that contains:

Lowercase English letters
Opening '(' and closing ')' parentheses

A string is considered valid if:

All opening parentheses '(' are closed properly by a matching ')'.
The parentheses are in the correct order and nesting.
Letters can appear anywhere and do not affect validity.

Return all possible valid strings that can be formed by removing the minimum number of invalid parentheses. The answer must be a list of unique strings, in any order.

Constraints:

$1 \leq$ s.length $\leq 25$
s consists of lowercase English letters and parentheses '(' and ')'.
There will be at most 20 parentheses in s. ...