C :

 #include <stdio.h>
  #include <string.h>
 
 int main()
  {
      int n, i, j;
     scanf("%d", &n);
     char str[20000] = "0", s[20000];
     while (n--)
    {
         for (i = 0, j = 0; str[i]; i++, j++)
        {
             if (str[i] == '0')
            {
                 s[j] = '1';
             }
            else if (str[i] == '1')
           {
                 s[j] = '0';
                 s[++j] = str[i];
             }            
        }
         s[j] = '\0';
        strcpy(str, s);
    }
    puts(str);
    return 0;
}

C++ :

#include<bits/stdc++.h>
using namespace std;
int main(){
	string s = "0",t;
	int n,i,j;
	cin>>n;
	for(i=1;i<=n;i++){
		t = "";
		for(j=0;j<s.size();j++){
			if(s[j]=='0'){
				t = t + '1'; 
			}else if(s[j]=='1'){
				t = t + "01";
			}
		} 
		s = t;
	}
	cout<<s;
	return 0;
}

Java :

import java.util.Scanner;
 
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		String a = "0";
		while (n > 0) {
			String b = "";
			for (int i = 0; i < a.length(); i++) {
				if (a.charAt(i) == '0')
					b += "1";
				else if (a.charAt(i) == '1')
					b += "01";
			}
			a = b;
			n--;
		}
		System.out.println(a);
	}
}