Hi this question was asked in my interview. 
Ans is :

a=a+b;
b=a-b;
a=a-b;

Any other solution?

use xor to swap

a = a^b
b= a^b
a= a^b

Ans is :

a=a*b;
b=a/b;
a=a/b;

a=5,b=10
a=a-b;
b=a+b;
a=a+b;

Only #2 will work.  The others may over/under flow.

a=a+b;
b=a-b;
a=a-b

b=a-b;
a=a-b;
b=a+b;

2 one will work correct

plz check 3 aswer is perfect, 2 one mat be wrong in some cases

a=b-a+(b=a);

Only the Xor answer (#2) is correct (in cases where the 
variables are the same size).  With all the other answers, 
you could run into over/under flow problems.
A = 01111111
B = 01111101
             
A = A^B = 00000010  
B = A^B = 01111111 (Original A)
A = A^B = 01111101 (Original B)

a=a*b/(b=a);

Answer no 12 is wrong. the logic fails when a=0;

x = x + y; 

    y = x - y; 

    x = x - y;

NOne of the answer is correct except the 2ND one.....Please 
don't give wrong answers.

x = y | (y=x);

Answer #16 is totally wrong it fails 
when x>y and many other situation.

a=(a>b?a:b)

using ref keyword we can swap 2 number's

emaple
class swap
{
public void add(ref int a,ref int b)
{
int z;
z=a;
a=b;
b=z;
}
}
class swapdemo
{
static void main()
{
int x=10,y=5;
ankush obj=new ankush();
Console.writeline{"befor value's"+a+","+b};
obj.swap(ref x, ref y);
Console.writeline("after values"+a+","+b);
}
}

Using Assembly language( Using Accumulator)...Without using 
any arithmatic...without using any Pointer...without 
declaring third varible

int a = 20;
int b = 10;
   __asm
  {
	  mov EAX,b
	  push EAX
	  mov EAX,a
	  mov b,EAX
	  pop EAX
	  mov a,EAX
  }

The first two answers are correct. Third will FAIL in the 
case the second num is 0. The #19 one still uses a third 
variable.

using X-OR 
#define SWAP(x,y) x^=y^=x^=y

x = x ^ y --> x^=y -- (1)
y = y ^ x --> y^=x -- (2)
x = x ^ y --> x^=y -- (3)

(3) in (2) --> y^=x^=y -- (4)
(4) in (1) --> x^=y^=x^=y -- :-)

all togeather, he single line code

#define SWAP(x,y) x^=y^=x^=y

Thanks a lot guys. It is giving very good analysis.

i am not giving answer
all above answer for swaping integer type variables without 
third
i want answer in string type

a=a+b-(b=a);