Remove Outermost Parentheses — LeetCode 1021 Python Solution

The problem

A valid parentheses string is either empty "", "(" + A + ")", or A + B, where A and B are valid parentheses strings, and + represents string concatenation. For example, "", "()", "(())()", and "(()(()))" are all valid parentheses strings.

Example

Input

s = "(()())(())"

Output

"()()()"

Explanation

The input string is "(()())(())", with primitive decomposition "(()())" + "(())".

Python solution

class Solution:
    def removeOuterParentheses(self, s: str) -> str:
        ans = []
        cnt = 0
        for c in s:
            if c == '(':
                cnt += 1
                if cnt > 1:
                    ans.append(c)
            else:
                cnt -= 1
                if cnt > 0:
                    ans.append(c)
        return ''.join(ans)

Complexity

Pattern: Stack

When the most recent unresolved thing is the one that matters, use a stack. LeetCode 1021. Remove Outermost Parentheses is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Stack.

The stack guide has the Python template for the pattern and the 194 LeetCode problems that use it.

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