Lintcode 67 二叉树的中序遍历

描述：

给出一棵二叉树,返回其中序遍历

样例：

给出二叉树 {1,#,2,3},

返回 [1,3,2].

挑战：

你能使用非递归算法来实现么?

代码：

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Inorder in vector which contains node values.
     */
public:

    vector ans;

    vector inorderTraversal(TreeNode *root) {
        // write your code here
        
        if(root != NULL)
        {
            inorderTraversal(root->left);
            ans.push_back(root->val);
            inorderTraversal(root->right);
        }
        return ans;
        
    }
};