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;
        inorder(root,res);
        return res;
    }
    void inorder(TreeNode *root, vector<int>& res){
        if(!root) return ;
        if(root->left)
            inorder(root->left,res);
        res.push_back(root->val);
        if(root->right)
            inorder(root->right,res);
    }
};
 

/**
 * 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 *p = root;
        while(!s.empty()||p){
            if(p){
                s.push(p);
                p = p->left;
            }else{
                TreeNode *t = s.top();
                s.pop();
                res.push_back(t->val);
                p = t->right;
            }
        }
        return res;
    }
};