Binary Tree Inorder Traversa

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {    void InOrder(TreeNode *root,vector<int> &vec){        if(root==NULL)return;                InOrder(root->left,vec);        vec.push_back(root->val);        InOrder(root->right,vec);            }public:    vector<int> inorderTraversal(TreeNode *root) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        vector<int> vec;        InOrder(root,vec);        return vec;            }};