/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static String mixStrings(String str1, String str2) {
		if(str1.length() != str2.length()) return null;
		String temp = "";
		for(int i = 0; i < str1.length(); i++) {
			temp += str1.charAt(i);
			temp += str2.charAt(i);
		}
		return temp;
	}
 
	public static void main (String[] args) throws java.lang.Exception
	{
		System.out.println(mixStrings("aaa", "bbb"));
		// your code goes here
	}
}

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