6.43③  編寫遞歸算法，將二叉樹中所有結點的
左、右子樹相互交換。

要求實現下列函數：
void Exchange(BiTree &bt);
/* Exchange the left and right leaves of */
/* bitree whose root node is bt          */

二叉鏈表類型定義：
typedef struct BiTNode {
    TElemType data;
    BiTNode  *lchild, *rchild;
} BiTNode, *BiTree;
void Exchange(BiTree &bt)
/* Exchange the left and right leaves of */
/* bitree whose root node is bt          */
{   
    struct BiTNode *temp;
    temp=bt->rchild;
    bt->rchild=bt->lchild;
    bt->lchild=temp;
    if(bt->rchild) Exchange(bt->rchild);
    if(bt->lchild) Exchange(bt->lchild);

}