二叉树的遍历：先序，中序，后序，递归，非递归，层次遍历

二叉树遍历的递归写法：

#include <iostream>
#include <vector>
#include <algorithm>
#include <stack>
using namespace std;

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x):val(x), left(NULL), right(NULL) {}
};

// 先序遍历
void preOrder(TreeNode* root)
{
    if(root == NULL)
        return;
    cout << root->val << " ";
    preOrder(root->left);
    preOrder(root->right);
}

// 中序遍历
void inOrder(TreeNode* root)
{
    if(root == NULL)
        return;
    inOrder(root->left);
    cout << root->val << " ";
    inOrder(root->right);
}

// 后序遍历
void lastOrder(TreeNode* root)
{
    if(root == NULL)
        return;
    lastOrder(root->left);
    lastOrder(root->right);
    cout << root->val << " ";
}
#include <iostream>
#include <vec