1963. Minimum Number of Swaps to Make the String Balanced

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.

class Solution {
public:
    int minSwaps(string s) {
        vector<char> st;
        for(auto& ch : s) {
            if(!st.empty() and st.back() == '[' and ch == ']') st.pop_back();
            else st.push_back(ch);
        }
        return (st.size() / 2 + 1) / 2;
    }
};