Is Minimum Add to Make Parentheses Valid easy or hard?

This problem is generally rated Medium because it requires recognizing that stack simulation can be optimized to a greedy counter approach. The logic is simple once you understand parentheses balance, but identifying the O(1) space optimization is the key insight.

How to solve Minimum Add to Make Parentheses Valid in O(n)?

Scan the string and maintain a balance counter for open parentheses. Increment for '(' and decrement for ')'. If the counter becomes negative, add one required opening parenthesis and reset the balance. After the scan, add the remaining balance to the result because each unmatched '(' needs a closing ')'.

Minimum Add to Make Parentheses Valid Python or Java solution?

Both Python and Java implementations typically use the greedy balance method. Iterate through the string, track the number of open parentheses, and count how many insertions are required when unmatched closing brackets appear. The algorithm runs in O(n) time and constant space.

What is the best approach for Minimum Add to Make Parentheses Valid?

Greedy balance tracking is the best approach. Iterate through the string while maintaining a counter for unmatched opening parentheses. When a closing parenthesis appears without a matching open one, increment the required additions. This solution runs in O(n) time and O(1) space, which is optimal.

What data structure is used in Minimum Add to Make Parentheses Valid?

Two common approaches exist. A stack can simulate parentheses matching by pushing '(' and popping when encountering ')'. A more optimal solution replaces the stack with a simple integer balance counter using a greedy strategy.

What is the time complexity of Minimum Add to Make Parentheses Valid?

The optimal solution runs in O(n) time where n is the length of the string. Each character is processed once while updating a balance counter or stack. The greedy method uses O(1) space, while the stack-based method uses O(n) space in the worst case.

Is Minimum Add to Make Parentheses Valid asked at Google, Amazon, or Meta?

Parentheses validation and balancing problems appear frequently in interviews at companies like Amazon, Google, and Meta. Variants include Valid Parentheses, Minimum Remove to Make Valid Parentheses, and longest valid parentheses, all testing stack or greedy reasoning.

Description Solution Video 7:29

921. Minimum Add to Make Parentheses Valid

Medium74.7% acceptance7 companies

A parentheses string is valid if and only if:

It is the empty string,
It can be written as AB (A concatenated with B), where A and B are valid strings, or
It can be written as (A), where A is a valid string.

You are given a parentheses string s. In one move, you can insert a parenthesis at any position of the string.