給定一個二叉樹，返回它的中序 遍歷。

示例:

輸入: [1,null,2,3]
   1
    \
     2
    /
   3

輸出: [1,3,2]
進階: 遞迴演算法很簡單，你可以通過迭代演算法完成嗎？

/**
 * 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 {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int>res;
        stack<TreeNode *>s;
        TreeNode * cur=root;
        while(cur||!s.empty())
        {
            while(cur)
            {
                s.push(cur);
                cur = cur->left;
            }
            if(!s.empty())
            {
                cur = s.top();
                s.pop();
                res.push_back(cur->val);
                cur = cur->right;
            }
        }
        return res;
    }
};