LeetCode 94. Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes’ values.

For example:
Given binary tree [1,null,2,3],

              1                \                 2                /               3

return [1,3,2].

Note: Recursive solution is trivial, could you do it iteratively?

/** * 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>vec;    vector<int> inorderTraversal(TreeNode* root) {        if(root==NULL)return vec;        else{            if(root->left!=NULL&&root->right!=NULL){                inorderTraversal(root->left);                vec.push_back(root->val);                inorderTraversal(root->right);            }            else if(root->right!=NULL&&root->left==NULL){                vec.push_back(root->val);                inorderTraversal(root->right);             }            else if(root->right==NULL&&root->left!=NULL){                                inorderTraversal(root->left);                vec.push_back(root->val);            }            else{                vec.push_back(root->val);            }        }       return vec;     }};

不知道为啥这种题还是中等难度。