[LeetCode] Binary Tree Inorder Traversal

question:

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

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

解题思路：递归

    vector res;
    void inorder(TreeNode *root)
    {
        if(root == NULL)
            return;
        inorder(root -> left);
        res.push_back(root -> val);
        inorder(root -> right);
    }
    vector inorderTraversal(TreeNode* root) {
        inorder(root);
        return res;
    }