Minimum number of swaps required to sort an array | Set 2

Given an array of N distinct elements, find the minimum number of swaps required to sort the array.

Note: The problem is not asking to sort the array by the minimum number of swaps. The problem is to find the minimum swaps in which the array can be sorted.

Examples:

Input : arr[] = {4, 3, 2, 1}
Output : 2
Explanation : Swap index 0 with 3 and 1 with 2 to get the sorted array {1, 2, 3, 4}.

Input : arr[] = { 3, 5, 2, 4, 6, 8}
Output : 3

This problem is already discussed in the previous article using graph. In this article another approach to solve this problem is discussed which is slightly different from the cycle approach.

The idea is to create a vector of pair in C++ with first element as array values and second element as array indices. The next step is to sort the vector of pair according to the first element of the pair. After that traverse the vector and check if the index mapped with the value is correct or not, if not then keep swapping until the element is placed correctly and keep counting the number of swaps.

Below is the implementation of the above approach:

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// CPP program to find the minimum number 
// of swaps required to sort an array
// of distinct element
  
#include<bits/stdc++.h>
using namespace std;
  
// Function to find minimum swaps to 
// sort an array
int findMinSwap(int arr[] , int n)
{
    // Declare a vector of pair     
    vector<pair<int,int>> vec(n);
      
    for(int i=0;i<n;i++)
    {
        vec[i].first=arr[i];
        vec[i].second=i;
    }
  
    // Sort the vector w.r.t the first
    // element of pair
    sort(vec.begin(),vec.end());
  
    int ans=0,c=0,j;
  
    for(int i=0;i<n;i++)
    {   
        // If the element is already placed
        // correct, then continue
        if(vec[i].second==i) 
            continue;
        else
        {
            // swap with its respective index 
            swap(vec[i].first,vec[vec[i].second].first);
            swap(vec[i].second,vec[vec[i].second].second); 
        
          
        // swap until the correct 
        // index matches
        if(i!=vec[i].second)
            --i;
          
        // each swap makes one element
        // move to its correct index, 
        // so increment answer
        ans++;
    }
  
    return ans;
}
  
// Driver code
int main()
{
    int arr[] = {1, 5, 4, 3, 2};
      
    int n = sizeof(arr)/sizeof(arr[0]);
      
    cout<<findMinSwap(arr,n);
      
    return 0;

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Output:

2


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