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。