Write a C function that interchanges the top two numbers on the stack.

Difficulty level
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This exercise is mostly suitable for students 
int interchange(stack *s)
{
    element e1, e2;
     if(Top(*s, &e1) && Pop(s))
     {
         if(Top(*s, &e2) && Pop(s))
         {
            Push(s,e1);
            Push(s,e2);
         }
         else
            Push(s,e1);
     }
}
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