C++ Program To Sort By 3-way quick sort(DUTCH NATIONAL FLAG ALGORITHM)
Description::
The quicksort algorithm is to find an element as pivot to partition the array around pivot. The sub-arrays on left and right of the part of the pivot in the array are recursively solved.
But this can be optimized.
3-way partition based quicksort logic->
arr[left..i] < pivot
arr[i+1..j-1] = pivot
arr[j..right] > pivot
Code::
#include<iostream>
#include<algorithm>
using namespace std;
void partition(int arr[],int left,int right,int &i,int &j)
{
i=left-1,j=right;
int p=left-1,q=right;
int v=arr[right];
while(true)
{
while(arr[++i]<v);
while(v<arr[--j])
{
if(j==left)
break;
}
if(i>=j)
break;
swap(arr[i],arr[j]);
if(arr[i]==v)
swap(arr[++p],arr[i]);
if(arr[j]==v)
swap(arr[j],arr[--q]);
}
swap(arr[i],arr[right]);
j=i-1;
for(int k=left;k < p; k++, j--)
swap(arr[k],arr[j]);
i=i+1;
for(int k=right-1;k>q;k--,i++)
swap(arr[i],arr[k]);
}
/*
3-way partition based quicksort logic->
arr[left..i] < pivot
arr[i+1..j-1] = pivot
arr[j..right] > pivot
*/
void quicksort(int arr[],int left,int right)
{
if(right<=left)
return;
int i=0,j=0;
partition(arr,left,right,i,j);
quicksort(arr,left,j);
quicksort(arr,i,right);
}
int main()
{
int n;
cout<<"Enter the size::"<<endl;
cin>>n;
int arr[n];
cout<<"Enter the elements::"<<endl;
for(int i=0;i<n;i++)
cin>>arr[i];
cout<<endl<<"The stored values::"<<endl;
for(int i=0;i<n;i++)
cout<<arr[i]<<" ";
cout<<endl;
quicksort(arr,0,n-1);//'n-1' is passed as parameter as last index of array will be n-1
cout<<endl<<"The sorted values are::"<<endl;
for(int i=0;i<n;i++)
cout<<arr[i]<<" ";
cout<<endl;
return 0;
}
Output::
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