Minimum number of swaps to make the string balanced

Two Pointers String Stack Greedy

78.1% acceptance

Feb 25, 2026

2562

150

You are given a 0-indexed string s of even length n. The string consists of exactly n / 2 opening brackets '[' and n / 2 closing brackets ']'. A string is called balanced if and only if: It is the empty string, or It can be written as AB, where both A and B are balanced strings, or It can be written as [C], where C is a balanced string. You may swap the brackets at any two indices any number of times. Return the minimum number of swaps to make s balanced.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn min_swaps(s: String) -> i32 {
    let mut unmatched = 0;
    for ch in s.chars() {
      if ch == '[' {
        unmatched += 1;
      } else {
        if unmatched > 0 {
          unmatched -= 1;
        }
      }
    }
    // unmatched is the number of unmatched '[' which equals unmatched ']'
    // each swap fixes 2 unmatched pairs
    (unmatched + 1) / 2
  }
}