Minimum Swaps for Bracket Balancing.cpp

/*
Minimum Swaps for Bracket Balancing
===================================

You are given a string of 2N characters consisting of N ‘[‘ brackets and N ‘]’ brackets. A string is considered balanced if it can be represented in the for S2[S1] where S1 and S2 are balanced strings. We can make an unbalanced string balanced by swapping adjacent characters. Calculate the minimum number of swaps necessary to make a string balanced.

Input:
The first line of input contains an integer T denoting the number of test cases. Then T test cases follow. The first line of each test case contains an integer N denoting the length of the string.

The second line of each test case contains the string consisting of '[' and ']'.

Output:
Print the minimum number of swaps to make the string balanced for each test case in a new line.

Constraints:
1<= T <=100
1<= N <=100000

Example:

Input  : []][][
Output : 2
First swap: Position 3 and 4
[][]][
Second swap: Position 5 and 6
[][][]

Input  : [[][]]
Output : 0
String is already balanced.
*/

#include <bits/stdc++.h>
using namespace std;

int main()
{
  int t;
  cin >> t;
  while (t--)
  {
    int n;
    cin >> n;
    string s;
    cin >> s;

    vector<int> pos;
    for (int i = 0; i < s.length(); ++i)
      if (s[i] == '[')
        pos.push_back(i);

    int count = 0;
    int p = 0;
    long sum = 0;

    for (int i = 0; i < s.length(); ++i)
    {
      if (s[i] == '[')
      {
        ++count;
        ++p;
      }
      else if (s[i] == ']')
        --count;

      if (count < 0)
      {
        sum += pos[p] - i;
        swap(s[i], s[pos[p]]);
        ++p;
        count = 1;
      }
    }
    cout << sum << endl;
  }
  return 0;
}