The first two answers are correct. Third will FAIL in the 
case the second num is 0...Please do not  post wrong answer

first & third answers are correct.

i think first is right.

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


is this wrong?

#include<stdio.h>
main()
{
int a,b;
printf("Enter any two numbers\n");
scanf("%d%d",&a,&b);
printf("The values before swapping are\n%d %d\n",a,b);
swap(&a,&b);
printf("The values after swapping are\n%d %d\n",a,b);
getch();
}

swap(*x,*y)
{
int t;
t=*x;
*x=*y;
*y=t;
}

void main()
{
int a,b;
printf("Enter two number : ");
scanf("%d%d",&a,&b);
a=a+b;
b=a-b;
a=a-b;
printf("The swap number is : %d %d",a,b);
getch();
}

Sorry i dont know..??
only 1st ans is write......
so give me positive marking....
   Thank You...

if any one of the variables eithet A or B is negative or 
else both r  negatibe then how logic will vary in cobol?

($a,$b)=($b,$a)
....this is how we do in PERL....Ha Ha Ha pretty cool !!!!

And BELIEVE me it works not only for NUMBERS but also for
STRINGS, ARRAYS or any Data Structure or any garbage
value....this is mine Challenge.....

None of the any programming language can come close to PERL
in this much of SIMPLICITY and ROBUSTNESS....just one line
does the Magic....

Again I would say the Question itself is very silly one "How
to swap two variables, without using third variable
?"....and I see alot of stupid Answers posted here....

Everyone posting the answer is assuming that Question is
about swapping INTEGER NUMBERS....I would like to ask what
if I provide FLOATING POINT NUMBERS, NEGATIVE NUMBERS, REAL
NUMBERS, and surely it does not work for STRINGS....

That is why I say "Perl is immensely powerful. If you think
something can't be done, the problem is likely to be it is
beyond your ability, not that of Perl."

Welcome to the world of PERL....its more precious than PEARL....

#include<stdio.h>
#include<conio.h>
int main()
{
    int a,b;
     clrscr();
    printf("\nEnter two numbers:");
     scanf("%d%d",&a,&b);
     printf("\nThe numbers after swapping are %d  %d",b,a);
      getch();
      return 0;
}

only #2 is d right answer......XOR yields...perfect answers.....

1st and 2nd methods r right!!
dere iz some problem wid 3rd..if a=0.

18th method...how u guy give dis type of solution?? is it
ryte?? suppose a>b den??

void main()
{
  int a,b;
  clrscr();
  printf("\n Enter the values of a and b ");
  scanf(" %d %d ", &a,&b);
  a=a*b;
  b=a/b;
  a=a/b;
  printf("\n \n After swapping ----> a = %d \t b = %d",a,b);
  getch();
}

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

This is the example by using only one line to swap the two 
variables without using the third variable.

a=10;
b=20;
a=a+b;
b=a-b;
a=a-b;
a=20,b=10;

declare a fourth variable and use that.
I dont understand why you need to do any of the above ??? if fourth is a problem, declare fifth and so on...

if x=14, y=18
x=x+y;
now x=14+18=32;
y=x-y;
now y=32-18=14;
now again y=14
x=x-y;
now x=32-14=18
final answer is:
x=18, and y=14
u can try this formule by taking any values for x and y.

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

I am new in C/C++.
How to write the program for second answer. Does it
automatically convert the numbers in binary by using ^ sign.

:}

a[12]=name
b[20]=name
    Here the size 12 & 25 has no effect hence we can change
the size
a[]=name
b[]=name