Write a function that will interchange all left and right subtrees in a binary tree.

Difficulty level
This exercise is mostly suitable for students 
void interchange(Btree *B)
{
    Btree aux;
    if(*B)
    {
        aux=(*B)->left;
        (*B)->left  = (*B)->right;
        (*B)->right = aux;
        interchange(&((*B)->left));
        interchange(&((*B)->right));
    }
}
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