本题代码如下

void btreetoexp(tree t, char deep)
{
	if (t == NULL)
		return;
	else if (t->lchild == NULL && t->rchild == NULL)
		printf("%c", t->data);//输出操作数，不加括号
	else {
		if (deep > 1)
			printf("(");
		btreetoexp(t->lchild, deep + 1);
		printf("%c", t->data);
		btreetoexp(t->rchild, deep + 1);
		if (deep > 1)
			printf(")");
	}
}

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)//建树
{
	char ch;
	ch = getchar();
	if (ch == '#')
		*t = NULL;
	else
	{
		*t = (treenode*)malloc(sizeof(treenode));//分配空间
		(*t)->data = ch;
		(*t)->lchild = NULL;
		(*t)->rchild = NULL;
		buildtree(&((*t)->lchild));
		buildtree(&((*t)->rchild));
	}
}
void btreetoexp(tree t, char deep)
{
	if (t == NULL)
		return;
	else if (t->lchild == NULL && t->rchild == NULL)
		printf("%c", t->data);//输出操作数，不加括号
	else {
		if (deep > 1)
			printf("(");
		btreetoexp(t->lchild, deep + 1);
		printf("%c", t->data);
		btreetoexp(t->rchild, deep + 1);
		if (deep > 1)
			printf(")");
	}
}
int main() {
	tree t;
	buildtree(&t);
	btreetoexp(t, 1);//根的高度为1
	return 0;
}
//*+a##b##*c##-#d##
/*
		*
	+		*
a	  b   c    -
				  d
				  */