//后序线索，这种方法不容易想到
#include<iostream>
#include<cstring>
#include<cstdio>
#include<algorithm>using namespace std;struct TREE{int    val;TREE *ch[2];TREE *thread;//该节点的线索的下一个节点 TREE(){}TREE(int val){this->val = val;ch[0] = ch[1] = NULL;thread = NULL;}
};void buildT(TREE * &T){int x;scanf("%d", &x);if(x == -1) return ;T = new TREE(x);buildT(T->ch[0]);buildT(T->ch[1]);
}void postThread(TREE *T, TREE *pre){if(!T) return;postThread(T->ch[0], T);postThread(T->ch[1], T);if(pre){if(pre->ch[0] == T && pre->ch[1])//T是左子树 T->thread = pre->ch[1]; //T的下一个节点就是它的兄弟节点 else //T是右子树T->thread = pre;//T的下一个节点就是它 的父亲节点             }
}void printT_pre(TREE *T){//前序打印 if(!T) return ;printf("%d ", T->val);printT_pre(T->ch[0]);printT_pre(T->ch[1]);
}void printT_Thread(TREE *T){//后序线索打印 TREE *root = T;bool flag = true;//表明T所在的字数没有访问过 while(1){while(flag && T->ch[0]) T = T->ch[0];printf("%d ", T->val);if(T->thread->ch[0] == T || T->thread->ch[1] == T)flag = false;//如果 T没有兄弟节点了，就一直沿着它的父节点向上走 else flag = true;T = T->thread;if(T == root){//再次回到最顶端父节点时， 结束！ printf("%d ", T->val);           break;}}
}int main(){TREE *T = NULL;buildT(T);printT_pre(T);printf("\n");postThread(T, NULL);printT_Thread(T);printf("\n");return 0;
}

下面是基本按照同样的套路实现二叉树的先序，中序，后序的线索

//先序线索
#include<stdio.h>
#include<stdlib.h>
#define Thread 1
#define Tlink 0
typedef struct tree
{int d;struct tree* lchild;struct tree* rchild;int Ltag, Rtag;
}*TREE;void build_tree(TREE &T)
{int d;scanf("%d", &d);if(d!=-1){T=(TREE)malloc(sizeof(tree));T->d=d;T->lchild=T->rchild=NULL;T->Ltag=T->Rtag=Tlink;build_tree(T->lchild);build_tree(T->rchild);}
} TREE pre;void preThread_tree(TREE p)
{if(p){if(!p->lchild){p->lchild=pre;p->Ltag=Thread;}if(!pre->rchild){pre->rchild=p;pre->Rtag=Thread;}pre=p;if(p->Ltag==Tlink)//考虑到只有一个结点或两个节点情况， preThread_tree(p->lchild);//如果没有 if语句可能出现死循环 if(p->Rtag==Tlink)preThread_tree(p->rchild);}
}void print_preThr_tree(TREE T)
{TREE p=T;while(1){ while(p->Ltag==Tlink){printf("%d ", p->d);p=p->lchild;}printf("%d ", p->d);p=p->rchild;if(p==T) break;while(p->Rtag==Thread){printf("%d ", p->d);p=p->rchild;}}
}void print_tree(TREE T)
{if(T){printf("%d ", T->d);print_tree(T->lchild);print_tree(T->rchild);}
}int main()
{TREE T=NULL;build_tree(T);print_tree(T);//递归先序遍历 printf("\n");    pre=T;preThread_tree(T);//非递归先序遍历 pre->rchild=T;print_preThr_tree(T);return 0;
}

//中序线索
#include<stdio.h>
#include<stdlib.h>
#define Thread 1
#define Tlink 0
typedef struct tree
{int d;struct tree* lchild;struct tree* rchild;int Ltag, Rtag;
}*TREE;void build_tree(TREE &T)
{int d;scanf("%d", &d);if(d!=-1){T=(TREE)malloc(sizeof(tree));T->d=d;T->lchild=T->rchild=NULL;T->Ltag=T->Rtag=Tlink;build_tree(T->lchild);build_tree(T->rchild);}
} TREE pre;void inorThread_tree(TREE p)
{if(p){if(p->Ltag==Tlink)//当只有一个结点的时候，要加上这句话 inorThread_tree(p->lchild);if(!p->lchild){p->lchild=pre;p->Ltag=Thread;}if(!pre->rchild){pre->rchild=p;pre->Rtag=Thread;}pre=p;if(p->Rtag==Tlink)inorThread_tree(p->rchild);}
}/*
上述代码可以简化如下，pre的初值为NULL就好　　
void inorThread_tree(TREE p)
{if(p){  inorThread_tree(p->lchild);if(!p->lchild){p->lchild=pre;p->Ltag=Thread;}if(pre && !pre->rchild){pre->rchild=p;pre->Rtag=Thread;}pre=p;  inorThread_tree(p->rchild);}
}
*/void print_tree(TREE T){if(T){print_tree(T->lchild);printf("%d ", T->d);print_tree(T->rchild);}
}void print_inorThr_tree(TREE T)
{TREE p=T;while(1){while(p->Ltag==Tlink)p=p->lchild;printf("%d ", p->d);while(p->Rtag==Thread){p=p->rchild;printf("%d ", p->d);}p=p->rchild;if(p==T)break;}
}int main()
{TREE T=NULL;build_tree(T);print_tree(T);//递归中序遍历 printf("\n");    pre=T;inorThread_tree(T);//非递归中序遍历 pre->rchild=T;pre->Rtag=0;//保证只有一个结点的情况 print_inorThr_tree(T);return 0;
}

//后序线索
#include<stdio.h>
#include<stdlib.h>
#define Thread 1
#define Tlink 0
typedef struct tree
{int d;struct tree* lchild;struct tree* rchild;struct tree* rrchild;int Ltag, Rtag;
}*TREE;void build_tree(TREE &T)
{int d;scanf("%d", &d);if(d!=-1){T=(TREE)malloc(sizeof(tree));T->d=d;T->lchild=T->rchild=NULL;T->Ltag=T->Rtag=Tlink;build_tree(T->lchild);build_tree(T->rchild);}
} TREE pre;void postThread_tree(TREE p)
{if(p){if(p->Ltag==Tlink)postThread_tree(p->lchild);if(p->Rtag==Tlink)postThread_tree(p->rchild);if(!p->lchild){p->lchild=pre;p->Ltag=Thread;}if(!pre->rchild ){pre->rchild=p;pre->Rtag=Thread;} else {pre->rrchild=p;}pre=p;}
}void print_postThr_tree(TREE T)
{TREE p=T;while(p->Ltag==Tlink)p=p->lchild;while(1){ printf("%d ", p->d);if(p==T)break; if(p->Rtag == Thread)p=p->rchild;else p=p->rrchild;}
}void print_tree(TREE T)
{if(T){print_tree(T->lchild);print_tree(T->rchild);printf("%d ", T->d);}
}int main()
{TREE T=NULL;build_tree(T);print_tree(T);//递归后序遍历 printf("\n");    pre=T;postThread_tree(T);//非递归后序遍历 pre->rchild=T;pre->Rtag = Thread; print_postThr_tree(T); return 0;
}