• 二叉树的中序遍历，指首先遍历左节点，然后遍历中间节点，最后遍历右节点，按照这个顺序进行递归即可。

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode() : val(0), left(nullptr), right(nullptr) {}*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}* };*/
class Solution {
public:void traversal(TreeNode* cur, vector<int>& vec){if(cur == nullptr){return;}traversal(cur->left, vec);vec.push_back(cur->val);traversal(cur->right, vec);}vector<int> inorderTraversal(TreeNode* root) {vector<int> result;traversal(root, result);return result;}
};

• 二叉树的中序遍历的迭代方法，和前后序存在一点不同，原因在于处理节点的顺序，与入栈节点的顺序不同，中序遍历需要首先将树的最左侧节点入栈，然后再处理中间节点，然后处理中间节点的右节点的左节点。

/*** Definition for a binary tree node.* struct TreeNode {*     int val;*     TreeNode *left;*     TreeNode *right;*     TreeNode() : val(0), left(nullptr), right(nullptr) {}*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}* };*/
class Solution {
public:vector<int> inorderTraversal(TreeNode* root) {vector<int> result;stack<TreeNode*> st;TreeNode* cur = root;while(cur != nullptr || !st.empty()){if(cur != nullptr){st.push(cur);cur = cur->left;} else {cur = st.top();st.pop();result.push_back(cur->val);cur = cur->right;}}return result;}
};