Given a collection of numbers(unique elements), return all possible permutations. (https://www.interviewbit.com/problems/permutations/)

permutations_interviewBit.cpp

/*
Given a collection of numbers, return all possible permutations.

Example:

[1,2,3] will have the following permutations:

[1,2,3]
[1,3,2]
[2,1,3] 
[2,3,1] 
[3,1,2] 
[3,2,1]

NOTE::
No two entries in the permutation sequence should be the same.
For the purpose of this problem, assume that all the numbers in the collection are unique.

Approach ::
Lets say we are at index start then we can swap element at this index with any index>start and permute 
the elements starting from start+1 using recursion. You can swap back the elements at start and index 
in order to maintain the order for next recursive call.
*/
#include<bits/stdc++.h>
using namespace std;

void display_vector(vector<int> &arr )
{
    for(int i = 0 ; i<arr.size() ; i++)
        {
            cout<<arr[i]<<" " ;
        }
        cout<<"\n" ;
}

void display_vectorOfVector(vector<vector<int> > &arr )
{
    for(int i = 0 ; i<arr.size() ; i++)
        {
            for(int j = 0 ; j<arr[i].size() ; j++)
                cout<<arr[i][j]<<" " ;
            cout<<"\n" ;
        }
}
void permute_helper(vector<vector<int> > &res , vector<int> &A , int start)
{
    //base case
    if(start == A.size()-1)
    {
        res.push_back(A) ;
        return ;
    }

    //recursive case
    for(int i = start ; i<A.size(); i++)
    {
        swap(A[i] , A[start]) ;
        permute_helper(res, A , start+1 ) ;
        swap(A[i] , A[start]) ;
    }
}

vector<vector<int> > permute(vector<int> &A) {
    vector<vector<int> > res ;
    permute_helper(res , A , 0 ) ;
    return res ;
}

int main()
{
	vector<int> A ;
	int n ;
	cout<<"enter the number of elements :" ;
	cin>>n ;
	cout<<endl ;
	for(int i = 0 ; i < n ; i++)
	{
	    int temp ;
	    cout<<"enter element :" ;
	    cin>>temp ;
	    A.push_back(temp) ;
	    cout<<endl ;
	}
    vector< vector<int> > ans = permute(A) ;
	cout<<"Output of program ==>\n" ;
	display_vectorOfVector(ans) ;
	return 0 ;
}