LEETCODE: Binary Tree Inorder Traversal

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].

递归方法，教科书上的。好像还可以用栈来解决这个问题。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void traverse(TreeNode *root, vector<int> &results) {
        if(root == NULL) return;
        traverse(root->left, results);
        results.push_back(root->val);
        traverse(root->right, results);
    }
    vector<int> inorderTraversal(TreeNode *root) {
        vector<int> results;
        traverse(root, results);
        return results;
    }
};