二叉树的建立，前序、中序、后序遍历，层序遍历（递归方式）

typedef struct node
{
    char data;
    struct node * left;
    struct node * right;
}Treepoint;
//二叉树结点的一种定义方式

建立二叉树，利用递归方式
这里存储的数据是char 型的数据，以‘#’结束。

Treepoint * creatTree()
{
    char ch;
    Treepoint * treenode;
    cin>>ch;
    if(ch == '#')
        treenode = NULL;
    else
    {
        treenode = (Treepoint *)malloc(sizeof(Treepoint));
        treenode->data = ch;
        treenode->left = creatTree();
        treenode->right = creatTree();
    }
    return treenode;
}

先序遍历
先序遍历首先访问根结点然后遍历左子树，最后遍历右子树。在遍历左、右子树时，仍然先访问根结点，然后遍历左子树，最后遍历右子树。

void forder(Treepoint * treenode)
{
    if(treenode == NULL)
        return ;
    cout<<treenode->data;//这里根据需要加分隔符，也可不加
    forder(treenode->left);
    forder(treenode->right);
}

中序遍历
中序遍历首先遍历左子树，然后访问根结点，最后遍历右子树。在遍历左、右子树时，仍然先遍历左子树，再访问根结点，最后遍历右子树。

void morder(Treepoint * treenode)//中序遍历
{
    if(treenode == NULL)
        return ;
    morder(treenode->left);
    cout<<treenode->data;
    morder(treenode->right);
}

后序遍历
后序遍历首先遍历左子树，然后遍历右子树，最后访问根结点，在遍历左、右子树时，仍然先遍历左子树，然后遍历右子树，最后遍历根结点。

void lorder(Treepoint * treenode)//后序遍历
{
    if(treenode == NULL)
        return ;
    lorder(treenode->left);
    lorder(treenode->right);
    cout<<treenode->data;
}

层序遍历，类似于bfs，利用队列一层一层进行遍历

void LevelOrder(BinTree bt)//类似BFS
{
	BinTree p;
	queue<BinTree> q;
	if(bt==NULL)
	{
		return;
	}
	p=bt;
	q.push(bt);
	while(!q.empty())
	{
		p=q.front();
		q.pop();
		cout<<p->data<<" ";
		if(p->leftchild!=NULL)
		{
			q.push(p->leftchild);
		}
		if(p->rightchild!=NULL)
		{
			q.push(p->rightchild);
		}
	}
} 

完整代码

#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <queue>

using namespace std;

typedef struct node//树结点的定义
{
    char data;
    struct node * left;
    struct node * right;
}Treepoint;

Treepoint * creatTree()//建立树
{
    char ch;
    Treepoint * treenode;
    cin>>ch;
    if(ch == '#')
        treenode = NULL;
    else
    {
        treenode = (Treepoint *)malloc(sizeof(Treepoint));
        treenode->data = ch;
        treenode->left = creatTree();
        treenode->right = creatTree();
    }
    return treenode;
}

void forder(Treepoint * treenode)//先序遍历
{
    if(treenode == NULL)
        return ;
    cout<<treenode->data;//这里根据需要加分隔符，也可不加
    forder(treenode->left);
    forder(treenode->right);
}

void morder(Treepoint * treenode)//中序遍历
{
    if(treenode == NULL)
        return ;
    morder(treenode->left);
    cout<<treenode->data;
    morder(treenode->right);
}

void lorder(Treepoint * treenode)//后序遍历
{
    if(treenode == NULL)
        return ;
    lorder(treenode->left);
    lorder(treenode->right);
    cout<<treenode->data;
}

void Lorder(Treepoint * treenode)//层序遍历
{
    queue <Treepoint *> q;
    Treepoint * p = treenode;
    if(p == NULL)
        return ;
    q.push(p);
    while(!q.empty())
    {
        p = q.front();
        q.pop();
        cout<<p->data;
        if(p->left != NULL)
            q.push(p->left);
        if(p->right != NULL)
            q.push(p->right);
    }
}

int main()
{
    Treepoint * root = creatTree();
    forder(root);
    cout<<endl;
    morder(root);
    cout<<endl;
    lorder(root);
    cout<<endl;
    Lorder(root);
    return 0;
}