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？

遞迴：

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;
    }
};