算法入門：樹的遍歷

二叉樹的遍歷主要有前序遍歷，中序遍歷，後序遍歷，層序遍歷四種方式

• 前序遍歷：中左右
• 中序遍歷：左中右
• 後序遍歷：左右中

遞歸

#include<iostream>
#include<queue>
#include<vector>
#include<stack>
using namespace std;
struct TreeNode {
	int val;
	struct TreeNode *left;
	struct TreeNode *right;
	TreeNode(int x) :
		val(x), left(NULL), right(NULL) {
	}
};

void PreOrder(TreeNode *T)//前序遍歷
{
	if (!T)
		return;
	cout << T->val<<endl;
	PreOrder(T->left);
	PreOrder(T->right);
}

void InOrder(TreeNode *T)//中序遍歷
{
	if (!T)
	    return;
	InOrder(T->left);
	cout << T->val;
	InOrder(T->right);
}
 
void PostOrder(TreeNode *T)//後序遍歷
{
	if (!T)
		return;
	PostOrder(T->left);
	PostOrder(T->right);
	cout << T->val;
}

非遞歸

#include<iostream>
#include<queue>
#include<vector>
#include<stack>
using namespace std;
struct TreeNode {
	int val;
	struct TreeNode *left;
	struct TreeNode *right;
	TreeNode(int x) :
		val(x), left(NULL), right(NULL) {
	}
};

void PreOrder(TreeNode *T)//前序遍歷
{
    stack< pair<TreeNode *, bool> > s;
    TreeNode *root=T;
    s.push(make_pair(root, false));
    bool visited;
    while(!s.empty())
    {
        root = s.top().first;
        visited = s.top().second;
        s.pop();
        if(root == NULL)
            continue;
        if(visited)
        {
            path.push_back(root->val);
        }
        else
        {
            s.push(make_pair(root->right, false));
            s.push(make_pair(root->left, false));
            s.push(make_pair(root, true));
        }
    }
}

void InOrder(TreeNode *T)//中序遍歷
{
    stack< pair<TreeNode *, bool> > s;
    TreeNode *root=T;
    s.push(make_pair(root, false));
    bool visited;
    while(!s.empty())
    {
        root = s.top().first;
        visited = s.top().second;
        s.pop();
        if(root == NULL)
            continue;
        if(visited)
        {
            path.push_back(root->val);
        }
        else
        {
            s.push(make_pair(root->right, false));
            s.push(make_pair(root, true));
            s.push(make_pair(root->left, false));
        }
    }
}
 
void PostOrder(TreeNode *T)//後序遍歷
{
    stack< pair<TreeNode *, bool> > s;
    TreeNode *root=T;
    s.push(make_pair(root, false));
    bool visited;
    while(!s.empty())
    {
        root = s.top().first;
        visited = s.top().second;
        s.pop();
        if(root == NULL)
            continue;
        if(visited)
        {
            path.push_back(root->val);
        }
        else
        {
            s.push(make_pair(root, true));
            s.push(make_pair(root->right, false));
            s.push(make_pair(root->left, false));
        }
    }
}

void Levelorder(TreeNode *T)//層析遍歷
{
	if (!T)
		return;
	TreeNode *temp;
	queue<TreeNode*>q;
	q.push(T);
	while (!q.empty())
	{
		temp = q.front();
		cout << temp->val;
		if (temp->left)
			q.push(temp->left);
		if (temp->right)
			q.push(temp->right);
		q.pop();
	}
}