E94.二叉树的中序遍历

dfs, stack, https://leetcode.cn/problems/binary-tree-inorder-traversal/

思路：简单的 dfs

class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        dfs(root, res);
        return res;
    }

    void dfs(TreeNode* root, vector<int>& res) {
        if (!root) return;
        dfs(root->left, res);
        res.push_back(root->val);
        dfs(root->right, res);
    }
};