网上的博客写得很好了，直接贴网址。

https://blog.csdn.net/gatieme/article/details/51163010

另外，wikipedia上也写得很详细：

https://zh.wikipedia.org/wiki/%E6%A0%91%E7%9A%84%E9%81%8D%E5%8E%86

一、Leetcode 144. 二叉树的前序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        fun(res,root);
        return res;
    }
    void fun(vector<int> &res,TreeNode* root)
    {
        if(root==NULL) return;
        res.push_back(root->val);
        fun(res,root->left);
        fun(res,root->right);
    }
};

二、Leetcode 94. 二叉树的中序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        fun(root,res);
        return res;
        
    }
    void fun(TreeNode* root, vector<int> &res)
    {
        if(root==NULL) return;
        fun(root->left,res);
        res.push_back(root->val);
        fun(root->right,res);
    }
};

三、Leetcode  145. 二叉树的后序遍历

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> res;
        fun(res,root);
        return res;
    }
    void fun(vector<int> &res,TreeNode* root)
    {
        if(root==NULL) return;
        fun(res,root->left);
        fun(res,root->right);
        res.push_back(root->val);
    }
};