二叉樹的鏈式儲存時會有較多空間的浪費，當一顆有 n 個節點的二叉樹儲存共有2n個指標域，但是隻有n-1個被用到，所以剩下的n+1個都會被浪費掉，因此可以想一種辦法把剩餘的空間利用起來，線索二叉樹應運而生。 將二叉樹重新定義如下每個節點為下列形式 lchild ltag data rtag rchild 其中：ltag=0 時lchild指向左子女； ltag=1 時lchild指向前驅； rtag=0 時rchild指向右子女； rtag=1 時rchild指向後繼； 所以結構體可定義為：

typedef enum{
	Link,
	Thread
}PointerTag;

typedef struct Tree_Node{
	char ch;
	struct Tree_Node *left;
	struct Tree_Node *right;
	PointerTag ltag;
	PointerTag rtag;
	
}Tree;

 

對於線索二叉樹同樣是先建立二叉樹，這裡採用先序建立和普通二叉樹相似，如下

Tree* create(){
	char ch;
	Tree *tree;
	scanf("%c",&ch);
	if(ch == '*')
	tree = NULL;
	else
	{
		tree = (Tree*)malloc(sizeof(Tree));
		tree->ch = ch;
		tree->left = create();
		tree->right = create();
	}
	return tree;
}

接下來便是直接對二叉樹進行轉換處理，使其變為線索二叉樹，採用中序遍歷二叉樹的方法來建立線索二叉樹

void trans(Tree *tree){
	if(tree){
		trans(tree->left);
		if(!tree->left){
			tree->ltag = Thread;
			tree->left = pre;
		}
		if(pre&&!pre->right){
			pre->rtag = Thread;
			pre->right = tree;
		}
		pre = tree;
		trans(tree->right);
	}
}

然後對線索二叉樹遍歷輸出

void print(Tree* tree){
	
		while(tree){
			while(tree->ltag == Link){
				tree = tree->left;
				
			}printf("%c ",tree->ch);
			while(tree->rtag == Thread && tree->right != NULL){
				tree = tree->right;
				printf("%c ",tree->ch);
			}
			tree = tree->right;
		}
	
}
 

下面附上完整程式碼

#include <stdio.h>
#include <stdlib.h>

typedef enum{
	Link,
	Thread
}PointerTag;

typedef struct Tree_Node{
	char ch;
	struct Tree_Node *left;
	struct Tree_Node *right;
	PointerTag ltag;
	PointerTag rtag;
	
}Tree;


Tree *pre;


Tree* create(){
	char ch;
	Tree *tree;
	scanf("%c",&ch);
	if(ch == '*')
	tree = NULL;
	else
	{
		tree = (Tree*)malloc(sizeof(Tree));
		tree->ch = ch;
		tree->left = create();
		tree->right = create();
	}
	return tree;
}
void trans(Tree *tree){
	if(tree){
		trans(tree->left);
		if(!tree->left){
			tree->ltag = Thread;
			tree->left = pre;
		}
		if(pre&&!pre->right){
			pre->rtag = Thread;
			pre->right = tree;
		}
		pre = tree;
		trans(tree->right);
	}
}


void print(Tree* tree){
	
		while(tree){
			while(tree->ltag == Link){
				tree = tree->left;
				
			}printf("%c ",tree->ch);
			while(tree->rtag == Thread && tree->right != NULL){
				tree = tree->right;
				printf("%c ",tree->ch);
			}
			tree = tree->right;
		}
	
}

int main(){
	Tree *tree;
	tree = create();
	trans(tree);
	print(tree);
}