本题代码如下

void postorder(tree* t)
{struct treenode* stack[100];//初始化结构体数组int top = -1;//让栈顶指向-1treenode* p = *t;while (p || top != -1)//p不为空，并且栈不为空{if (p){top++;//p不为空，将p压入栈中stack[top] = p;p = p->lchild;//一直向左下遍历}else{p = stack[top];//p等于栈顶元素if (p->rchild && p->rchild->tag == 0)//右孩子不为空且未被访问过p = p->rchild;else//否则弹出结点并访问{p = stack[top];top--;printf("%c", p->data);p->tag = 1;//标记p被访问过p = NULL;}}}
}

完整测试代码

#include<stdio.h>
#include<stdlib.h>
typedef struct treenode
{char data;struct treenode* lchild, * rchild;int tag;
}treenode,*tree;
void buildtree(tree *t)
{char ch;ch = getchar();if (ch == '#')*t = NULL;else{*t = (treenode*)malloc(sizeof(treenode));(*t)->data = ch;(*t)->tag = 0;(*t)->lchild = NULL;(*t)->rchild = NULL;buildtree(&(*t)->lchild);buildtree(&(*t)->rchild);}
}
void postorder(tree* t)
{struct treenode* stack[100];//初始化结构体数组int top = -1;//让栈顶指向-1treenode* p = *t;while (p || top != -1)//p不为空，并且栈不为空{if (p){top++;//p不为空，将p压入栈中stack[top] = p;p = p->lchild;//一直向左下遍历}else{p = stack[top];//p等于栈顶元素if (p->rchild && p->rchild->tag == 0)//右孩子不为空且未被访问过p = p->rchild;else//否则弹出结点并访问{p = stack[top];top--;printf("%c", p->data);p->tag = 1;//标记p被访问过p = NULL;}}}
}
int main()
{tree t;buildtree(&t);postorder(&t);return 0;
}

用ABD##E##CF##G##

/*        A
    B        C
D      E   F     G    
*/