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We have to print all the combination of "abcdef". This can be easily done using recursion with backtracking.

Java?

can you please tell how to use recursion for this.

can you please tell how to use recursion for this.

We can do like this in java

public static ArrayList<String> permutation(String s)
	{
		if(s==null)
		return null;
		if(s.length()<=1)
		{
			ArrayList<String> a=new ArrayList<String>();
			a.add(s);
			return a;
		}
		char c=s.charAt(0);
		ArrayList<String> a2=permutation(s.substring(1,s.length()));
		for(String s2:a2)
		{
			for(int i=0;i<s2.length()+1;i++)
			{
				String s3=insert(s2,i,c);
				a.add(s3);
			}
		}
		return a;
	}
	public static String insert(String s,int i,char c)
	{
		String s1=s.substring(0,i);
		String s2=s.substring(i);
		String s3=s1+c+s2;
		return s3;
	}

Problem is to find all permutation of given string.
I will go with c++ as there is in-built function next_permutation to do this.

cplusplus.com/reference/algorithm/next_permutation/