94. 二叉树的中序遍历(LeetCode)(dfs+递归写法)

94. 二叉树的中序遍历

dfs写法

/**
 * 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> res;
    vector<int> inorderTraversal(TreeNode* root) {
        dfs(root);
        return res;
    }

    void dfs(TreeNode* root){
        if(root == NULL) return;
        dfs(root->left);
        res.push_back(root->val);
        dfs(root->right);
    } 
};

![](https://img2024.cnblogs.com/blog/3136068/202503/3136068-20250311213130885-1895522695.png)

递归写法

/**
 * 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> res;
    stack<TreeNode*> stk;

    vector<int> inorderTraversal(TreeNode* root) {
        while(root || stk.size()){
            while(root){
                stk.push(root);
                root = root->left;
            }
            root = stk.top();
            stk.pop();
            res.push_back(root->val);
            root = root->right;
        }
    
        return res;
    }

};