How to Swap Two Numbers without Temp or Third variable in Java - Interview Question

How to swap two numbers without using temp or third variable is common interview question not just on Java interviews but also on C and C++ interviews. It is also a good programming questions for freshers. This question was asked to me long back and didn't had any idea about how to approach this question without using temp or third variable, may be lack of knowledge on bitwise operators in Java or may be it didn't click at that time. Given some time and trial error, I eventually come out with a solution with just arithmetic operator but interviewer was keep asking about other approaches of swapping two variables without using temp or third variable. Personally, I liked this question and included in list of my programming interview question because of its simplicity and some logical work, it force you to do. When learned bit-wise operation in Java I eventually find another way of swapping two variables without third variable, which I am going to share with you guys.

Swapping two numbers without using temp variable in Java

If you have ever heard this question, then you must be familiar with this approach of swapping numbers without using temp variable. If you are hearing it first time, then try it yourself, its a good programming exercise for absolute first timer. By the way, here is the code example of swapping two numbers without using temp variable and using arithmetic operator in Java:

System.out.println("value of a and b before swapping, a: " + a +" b: " + b);

//swapping value of two numbers without using temp variable

a = a+ b; //now a is 30 and b is 20

b = a -b; //now a is 30 but b is 10 (original value of a)

a = a -b; //now a is 20 and b is 10, numbers are swapped

System.out.println("value of a and b after swapping, a: " + a +" b: " + b);

value of a and b before swapping, a: 10 b: 20

value of a and b after swapping, a: 20 b: 10

Swapping two numbers without using temp variable in Java with bitwise operator

Bitwise operators can also be used to swap two numbers without using third variable. XOR bitwise operator returns zero if both operand is same i.e. either  0 or 1 and returns 1 if both operands are different e.g. one operand is zero and other is one. By leveraging this property, we can swap two numbers in Java. Here is code example of swapping two numbers without using temp variable in Java using XOR bitwise operand:

A       B       A^B (A XOR B)

0       0       0 (zero because operands are same)

0       1       1

1       0       1 (one because operands are different)

1       1       0

int a = 2; //0010 in binary

int b = 4; //0100 in binary

      

System.out.println("value of a and b before swapping, a: " + a +" b: " + b);

       

//swapping value of two numbers without using temp variable and XOR bitwise operator     

a = a^b; //now a is 6 and b is 4

b = a^b; //now a is 6 but b is 2 (original value of a)

a = a^b; //now a is 4 and b is 2, numbers are swapped

      

System.out.println("value of a and b after swapping using XOR bitwise operation, a: " + a +" b: " + b);

value of a and b before swapping, a: 2 b: 4

value of a and b after swapping using XOR bitwise operation, a: 4 b: 2

Swapping two numbers without using temp variable in Java with division and multiplication

There is another, third way of swapping two numbers without using third variable, which involves multiplication and division operator. This is similar to first approach, where we have used + and - operator for swapping values of two numbers. Here is the code example to swap tow number without using third variable with division and multiplication operators in Java :

System.out.println("value of a and b before swapping, a: " + a +" b: " + b);

//swapping value of two numbers without using temp variable using multiplication and division

a = a*b; //now a is 18 and b is 3

b = a/b; //now a is 18 but b is 6 (original value of a)

a = a/b; //now a is 3 and b is 6, numbers are swapped

System.out.println("value of a and b after swapping using multiplication and division, a: " + a +" b: " + b);

value of a and b before swapping, a: 6 b: 3

value of a and b after swapping using multiplication and division, a: 3 b: 6

That's all on 3 ways to swap two variables without using third variable in Java. Its good to know multiple ways of swapping two variables without using temp or third variable to handle any follow-up question. Swapping numbers using bitwise operator is the fastest among three, because it involves bitwise operation. It’s also great way to show your knowledge of bitwise operator in Java and impress interviewer, which then may ask some question on bitwise operation. A nice trick to drive interview on your expert area.

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