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引言:
遍历二叉树:指按某条搜索路径巡访二叉树中每个结点,使得每个结点均被访问一次,而且仅被访问一次。
 除了层次遍历外,二叉树有三个重要的遍历方法:先序遍历、中序遍历、后序遍历。
 1、递归算法实现先序、中序、后序遍历:
(1)先序遍历:
void PreOrderTraverse(BiTree T)
{if(T){cout<<T->data;PreOrderTraverse(T->lchild);PreOrderTraverse(T->rchild);}
}
 
(2)中序遍历:
void InOrderTraverse(BiTree T)
{   if(T){InOrderTraverse(T->lchild);cout<<T->data;InOrderTraverse(T->rchild);}
}  
(3)后序遍历
void PostOrderTraverse(BiTree T)
{   if(T){  PostOrderTraverse(T->lchild); PostOrderTraverse(T->rchild); cout<<T->data;   }
} 
 
2.非递归算法实现先序、中序、后序遍历:
采用非递归算法则需要利用栈来实现对二叉树的遍历:
 (1)先序遍历非递归算法
void  PreOrder_non_recursion(BiTree T)//先序遍历的非递归算法 
{LinkStack S;InitStack (S);   BiTree p,q;p=T;while(p||!StackEmpty(S)){if(p){Push(S,*p); cout<<p->data; //访问根节点 p=p->lchild;   //遍历左子树 }else{Pop(S,*q);p=q->rchild;   //遍历右子树 }}
}
 
(2)中序遍历非递归算法
void  InOrder_non_recursion(BiTree T)//中序遍历的非递归算法 
{LinkStack S;InitStack (S);   BiTree p;    BiTree q; p=T;while(p||!StackEmpty(S)){if(p){Push(S,*p); p=p->lchild;   //遍历左子树 }else{Pop(S,*q);cout<<q->data; //访问根节点 p=q->rchild;   //遍历右子树 }}
}
 
(3)后序遍历非递归算法
 (采用非递归算法实现对二叉树的后序遍历,会稍微复杂一些,本算法借用了两个栈结构)
void  PostOrder_non_recursion(BiTree T)//后序遍历的非递归算法 
{LinkStack l_S,r_S;InitStack (l_S);InitStack (r_S);BiTree p,q;    p=T;Push(l_S,*p);while(!StackEmpty(l_S)){Pop(l_S, *q);Push(r_S,*q);if(q->lchild){Push(l_S, *q->lchild);}if(q->rchild){Push(l_S,*q->rchild);}}while(!StackEmpty(r_S)){Pop(r_S,*q);cout<<q->data;}
}
 
3.完整代码
1、采用按照先序遍历的顺序建立二叉链表,用‘#’表示空树。如图所示:
 
 2、先序遍历的递归与非递归算法的对比:
#include<iostream>
#define OK 1
#define ERROR 0
#define OVERFLOW -2
using namespace std;
typedef char TElemType;
typedef int Status;typedef struct BiTNode{  //二叉树的存储结构TElemType   data;	// 数据域struct  BiTNode *lchild; //左孩子指针struct  BiTNode *rchild; //右孩子指针
}BiTNode, *BiTree;typedef struct StackNode {  //栈的存储结构BiTNode data;       //栈数据元素类型为树结点型 struct StackNode *next;
} StackNode, *LinkStack;Status InitStack(LinkStack &S) { //栈初始化S = NULL;return OK;
}Status Push(LinkStack &S, BiTNode e) { //入栈LinkStack p;p = new StackNode; //生成新结点if (!p) {return OVERFLOW;}p->data = e; //将新结点数据域置为ep->next = S; //将新结点插入栈顶S = p; //修改栈顶指针为preturn OK;
}Status Pop(LinkStack &S, BiTNode &e) {  //出栈LinkStack p;if (S == NULL)return ERROR; //栈空e = S->data; //将栈顶元素赋给ep = S; //用p临时保存栈顶元素空间,以备释放S = S->next; //修改栈顶指针delete p; //释放原栈顶元素的空间return OK;
}bool StackEmpty(LinkStack S) {  //判断是否空栈if (!S)return true;return false;
}void CreateBiTree_PreOrder(BiTree &T){ //以先序次序创建二叉树 char ch; cin>>ch;if(ch=='#')T=NULL; else{T=new BiTNode;  //生成根结点T->data=ch; //根结点的数据域置为chCreateBiTree_PreOrder(T->lchild);//构造左子树CreateBiTree_PreOrder(T->rchild); //构造右子树}}void PreOrder(BiTree T){   //先序遍历的递归递归算法if(T){cout<<T->data;PreOrder(T->lchild);PreOrder(T->rchild);}
}void  PreOrder_non_recursion(BiTree T)//先序遍历的非递归算法 
{LinkStack S;InitStack (S);   BiTree p,q;p=T;while(p||!StackEmpty(S)){if(p){Push(S,*p); cout<<p->data; //访问根节点 p=p->lchild;   //遍历左子树 }else{Pop(S,*q);p=q->rchild;   //遍历右子树 }}
}int main() {BiTree T;cout<<"以先序次序创建二叉链表,以#表示空子树:"<<endl;CreateBiTree_PreOrder(T);cout<<"先序序列(递归算法):"; PreOrder(T); cout<<"\n先序序列(非递归算法):"; PreOrder_non_recursion(T);return 0;
} 
实验结果:
 
 3、中序遍历的递归与非递归算法的对比:
#include<iostream>
#define OK 1
#define ERROR 0
#define OVERFLOW -2
using namespace std;
typedef char TElemType;
typedef int Status;typedef struct BiTNode{  //二叉树的存储结构TElemType   data;	// 数据域struct  BiTNode *lchild; //左孩子指针struct  BiTNode *rchild; //右孩子指针
}BiTNode, *BiTree;typedef struct StackNode {  //栈的存储结构BiTNode data;       //栈数据元素类型为树结点型 struct StackNode *next;
} StackNode, *LinkStack;Status InitStack(LinkStack &S) { //栈初始化S = NULL;return OK;
}Status Push(LinkStack &S, BiTNode e) { //入栈LinkStack p;p = new StackNode; //生成新结点if (!p) {return OVERFLOW;}p->data = e; //将新结点数据域置为ep->next = S; //将新结点插入栈顶S = p; //修改栈顶指针为preturn OK;
}Status Pop(LinkStack &S, BiTNode &e) {  //出栈LinkStack p;if (S == NULL)return ERROR; //栈空e = S->data; //将栈顶元素赋给ep = S; //用p临时保存栈顶元素空间,以备释放S = S->next; //修改栈顶指针delete p; //释放原栈顶元素的空间return OK;
}bool StackEmpty(LinkStack S) {  //判断是否空栈if (!S)return true;return false;
}void CreateBiTree_PreOrder(BiTree &T){ //以先序次序创建二叉树 char ch; cin>>ch;if(ch=='#')T=NULL; else{T=new BiTNode;  //生成根结点T->data=ch; //根结点的数据域置为chCreateBiTree_PreOrder(T->lchild);//构造左子树CreateBiTree_PreOrder(T->rchild); //构造右子树}}void InOrder(BiTree T){   //中序遍历的递归递归算法if(T){InOrder(T->lchild);cout<<T->data;InOrder(T->rchild);}
}void  InOrder_non_recursion(BiTree T)//中序遍历的非递归算法 
{LinkStack S;InitStack (S);   BiTree p;    BiTree q; p=T;while(p||!StackEmpty(S)){if(p){Push(S,*p); p=p->lchild;   //遍历左子树 }else{Pop(S,*q);cout<<q->data; //访问根节点 p=q->rchild;   //遍历右子树 }}
}int main() {BiTree T;cout<<"以先序次序创建二叉链表,以#表示空子树:"<<endl;CreateBiTree_PreOrder(T);cout<<"中序序列(递归算法):"; InOrder(T); cout<<"\n中序序列(非递归算法):"; InOrder_non_recursion(T);return 0;
} 
实验结果:
 
4、后序遍历的递归与非递归算法的对比:
#include<iostream>
#define OK 1
#define ERROR 0
#define OVERFLOW -2
using namespace std;
typedef char TElemType; 
typedef int Status;typedef struct BiTNode{  //二叉树的存储结构TElemType   data;	// 数据域struct  BiTNode *lchild; //左孩子指针struct  BiTNode *rchild; //右孩子指针
}BiTNode, *BiTree;typedef struct StackNode {  //栈的存储结构BiTNode data;       //栈数据元素类型为树结点型 struct StackNode *next;
} StackNode, *LinkStack;Status InitStack(LinkStack &S) { //栈初始化S = NULL;return OK;
}Status Push(LinkStack &S, BiTNode e) { //入栈LinkStack p;p = new StackNode; //生成新结点if (!p) {return OVERFLOW;}p->data = e; //将新结点数据域置为ep->next = S; //将新结点插入栈顶S = p; //修改栈顶指针为preturn OK;
}Status Pop(LinkStack &S, BiTNode &e) {  //出栈LinkStack p;if (S == NULL)return ERROR; //栈空e = S->data; //将栈顶元素赋给ep = S; //用p临时保存栈顶元素空间,以备释放S = S->next; //修改栈顶指针delete p; //释放原栈顶元素的空间return OK;
}bool StackEmpty(LinkStack S) {  //判断是否空栈if (!S)return true;return false;
}void CreateBiTree_PreOrder(BiTree &T){ //以先序次序创建二叉树 char ch; cin>>ch;if(ch=='#')T=NULL; else{T=new BiTNode;  //生成根结点T->data=ch; //根结点的数据域置为chCreateBiTree_PreOrder(T->lchild);//构造左子树CreateBiTree_PreOrder(T->rchild); //构造右子树}}void PostOrder(BiTree T){   //后序遍历的递归递归算法if(T){PostOrder(T->lchild);PostOrder(T->rchild);cout<<T->data;}
}void  PostOrder_non_recursion(BiTree T)//后序遍历的非递归算法 
{LinkStack l_S,r_S;InitStack (l_S);InitStack (r_S);BiTree p,q;    p=T;Push(l_S,*p);while(!StackEmpty(l_S)){Pop(l_S, *q);Push(r_S,*q);if(q->lchild){Push(l_S, *q->lchild);}if(q->rchild){Push(l_S,*q->rchild);}}while(!StackEmpty(r_S)){Pop(r_S,*q);cout<<q->data;}
}int main() {BiTree T;cout<<"以先序次序创建二叉链表,以#表示空子树:"<<endl;CreateBiTree_PreOrder(T);cout<<"中序序列(递归算法):"; PostOrder(T); cout<<"\n中序序列(非递归算法):"; PostOrder_non_recursion(T);return 0;
} 
实验结果:
 
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