94. 二叉树的中序遍历 迭代 二叉树 栈

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？

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