Binary Tree Inorder Traversal @LeetCode

PROBLEM :

Given a binary tree, return the inorder traversal of its nodes' values.

For example:
Given binary tree [1,null,2,3],
   1
    \
     2
    /
   3
return [1,3,2].

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SIMPLE c++ IMPLEMENTATION :( Recursive Solution )
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 /**
 * 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> vec ;
        if(root==NULL)
            return vec ;
           
        InorderRecursive(root,vec) ;
        return vec ;
    }
   
    void InorderRecursive(TreeNode* root,vector<int> &vec){
        if(root==NULL)
            return ;
        InorderRecursive(root->left,vec) ;
        vec.push_back(root->val) ;
        InorderRecursive(root->right,vec) ;
    }
};

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SIMPLE c++ IMPLEMENTATION :( Iterative Solution )
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/**
 * 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> vec ;
       
        if(root==NULL)
            return vec ;
           
        stack<TreeNode*> sta ;
       
        while(true){
            if(root){
                sta.push(root) ;
                root=root->left ;
            }
            else
            {
                if(sta.empty())
                    break ;
                   
                TreeNode* node ;
                node=sta.top() ;
                sta.pop() ;
               
                vec.push_back(node->val) ;
                root=node->right ;
            }
        }
        return vec ;
    }
};

--------------------------------------------------------------------------------
SIMPLE c++ IMPLEMENTATION :( Recursive Solution - Morris Traversal)
--------------------------------------------------------------------------------

 /**
 * 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> vec ;
       
        if(root==NULL)
            return vec ;
           
        TreeNode* curr,*prev ;
        curr=root ;
       
        while(curr!=NULL){
            if(curr->left==NULL){
                vec.push_back(curr->val) ;
                curr=curr->right ;
            }
            else{
                prev=curr->left ;
               
                while(prev->right&&prev->right!=curr)
                    prev=prev->right ;
                   
                if(prev->right==NULL){
                    prev->right=curr ;
                    curr=curr->left ;
                }
                else{
                    prev->right=NULL ;
                    vec.push_back(curr->val) ;
                    curr=curr->right ;
                }
            }
        }
        return vec ;
    }
};

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