给定一个二叉树，返回它的中序 遍历。

示例:

输入: [1,null,2,3] 1 \ 2 / 3 输出: [1,3,2]

进阶: 递归算法很简单，你可以通过迭代算法完成吗？

递归：

class Solution {

public:

    vector<int> res;

    vector<int> inorderTraversal(TreeNode* root)

    {

        if(root == NULL)

            return res;

        if(root ->left != NULL)

        {

            inorderTraversal(root ->left);

        }

        res.push_back(root ->val);

        if(root ->right != NULL)

        {

            inorderTraversal(root ->right);

        }

        return res;

    }

};

迭代：

class Solution {

public:

    vector<int> inorderTraversal(TreeNode* root)

    {

        vector<int> res;

        stack<TreeNode*> s;

        TreeNode *cur = root;

        while(!s.empty() || cur != NULL)

        {

            if(cur != NULL)

            {

                s.push(cur);

                cur = cur ->left;

            }

            else

            {

                cur = s.top();

                s.pop();

                res.push_back(cur ->val);

                cur = cur ->right;

            }

        }

        return res;

    }

};