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].



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

        before = inorderTraversal(root->right, before);
        return before;
    }
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> result;
        return inorderTraversal(root, result);
    }
};