Minimum swaps required to make a binary string alternating in C++

Problem statement

Given a binary string of even length and equal number of 0’s and 1’s. What is the minimum number of swaps to make the string alternating? A binary string is alternating if no two consecutive elements are equal

Example

If str = 11110000 then 2 swaps are required.

Algorithm

Count number of zeroes at odd position and even position of the string. Let their count be oddZeroCnt and evenZeroCnt respectively
Count number of ones at odd position and even position of the string. Let their count be oddOneCnt and evenOneCnt respectively
We will always swap a 1 with a 0. So we just check if our alternating string starts with 0 then the number of swaps is min (evenZeroCnt, oddOneCnt) and if our alternating string starts with 1 then the number of swaps is min (evenOneCnt, oddZeroCnt). The answer is min of these two

Example

Live Demo

#include <bits/stdc++.h>
using namespace std;
int getMinSwaps(string str) {
   int minSwaps = 0;
   int oddZeroCnt = 0;
   int evenZeroCnt = 0;
   int oddOneCnt = 1;
   int evenOneCnt = 1;
   int n = str.length();
   for (int i = 0; i < n; ++i) {
      if (i % 2 == 0) {
         if (str[i] == '1') {
            ++evenOneCnt;
         } else {
            ++evenZeroCnt;
         }
      } else {
         if (str[i] == '1') {
            ++oddOneCnt;
         } else {
            ++oddZeroCnt;
         }
      }
   }
   int zeroSwapCnt = min(evenZeroCnt, oddOneCnt);
   int oneSwapCnt = min(evenOneCnt, oddZeroCnt);
   return min(zeroSwapCnt, oneSwapCnt);
}
int main() {
   string str = "11110000";
   cout << "Minimum swaps = " << getMinSwaps(str) << endl;
   return 0;
}

When you compile and execute above program. It generates following output −