Binary Tree Inorder Traversal -- LeetCode 94

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

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,3,2].

Solution 1:

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

Solution 2: