本题代码如下

void swap(tree* t)
{if (*t){treenode* temp = (*t)->lchild;(*t)->lchild = (*t)->rchild;(*t)->rchild = temp;swap(&(*t)->lchild);swap(&(*t)->rchild);}
}

完整测试代码

#include<stdio.h>
#include<stdlib.h>
typedef struct treenode
{char data;struct treenode* lchild, * rchild;
}treenode,*tree;
void buildtree(tree* t)
{char ch;ch = getchar();if (ch =='#')*t = NULL;else{*t = (treenode*)malloc(sizeof(treenode));(*t)->data = ch;buildtree(&((*t)->lchild));buildtree(&((*t)->rchild));}
}
void swap(tree* t)
{if (*t){treenode* temp = (*t)->lchild;(*t)->lchild = (*t)->rchild;(*t)->rchild = temp;swap(&(*t)->lchild);swap(&(*t)->rchild);}
}
void disp(tree* t)
{if (*t){printf("%c", (*t)->data);disp(&(*t)->lchild);disp(&(*t)->rchild);}
}
int main()
{tree t;buildtree(&t);printf("交换过后的二叉树为（前序序列）：");swap(&t);disp(&t);return 0;
}

用ABD##E##CF##G##测试