LeetCode Entry

921. Minimum Add to Make Parentheses Valid

06.10.2026 medium 2026 kotlin rust

Min insertions to balance braces

921. Minimum Add to Make Parentheses Valid medium substack youtube

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Problem TLDR

Min insertions to balance braces

Intuition

Calculate count of times balance tries to go negative plus final balance

Approach

• shorter code: pop each () until its gone, the final length is unbalanced

Complexity

• Time complexity: \(O(n^2)\)

• Space complexity: \(O(n)\)

Code

    fun minAddToMakeValid(s: String): Int =
        if ("()" in s) minAddToMakeValid(s.replace("()", "")) else s.length

    pub fn min_add_to_make_valid(mut s: String) -> i32 {
        while s.contains("()") { s = s.replace("()", "") } s.len() as _
    }